At 4:03 p.m. ET on Wednesday, during a second-set tiebreak on Arthur Ashe Stadium in which Mirra Andreeva held two match points, one second of selling walked Coco Gauff's contract to win the US Open down to 2¢. The biggest fill in that second was 2,032 contracts at that price. Gauff saved both match points, won the tiebreak 9-7 and took the third set 6-2. At 4:33 p.m., with the third set nearly done, a single buyer took 9,821 contracts of the same title line at 29¢. As of September 9, 2026, at 6:07 p.m. ET, Kalshi's tournament board has Gauff at 27¢ bid, 28¢ ask, second on the board behind Aryna Sabalenka. The brand-new match market for Thursday's semifinal has Gauff at 53¢ bid, 54¢ ask, against Elena Rybakina at 46¢ bid, 47¢ ask.
Those two numbers, 27 and 53, are the ones this page is about. The match market is just over an hour old and has traded about 5,000 contracts. The title board has traded more than 850,000 contracts on Gauff alone, so the title board is the market with the opinion. Divide one by the other and Kalshi is saying that if Gauff beats Rybakina, she is a coin flip to win Saturday's final. The same arithmetic on Rybakina, on Sabalenka and on Jessica Pegula tells you whether Thursday's two semifinals are priced consistently with each other, and my read before the evidence is that they are, within a point and a half on a board whose spread is five cents, which means the edge a reader is hoping to find between the match line and the title line is not there. What is there is a wall of 101,724 contracts, and I will get to who is standing behind it.
How Gauff And Rybakina Got To Thursday
Both women lost the first set on Wednesday, and both had their title contracts cut roughly in half before the second set was over. The tape on each is worth reading, because it is the market pricing the same two players it now has to price against each other.
Rybakina played first. Zheng Qinwen broke her for 5-3 in the opening set and took it 6-3, and by 12:34 p.m. Rybakina's contract to win the tournament, which had printed 19¢ and 20¢ all morning, was at 11¢. Then she won the second set 6-1 in 38 minutes, and the buying started before the third set was decided: 5,447 contracts at 16¢ at 12:45 p.m., and at 1:48 p.m., late in the third set, a single fill of 10,148 contracts at 19¢. She closed it out 3-6, 6-1, 6-4 in 2 hours and 14 minutes, from a break point down at 3-4 in the third, after which she won six straight points and the last three games. The WTA's report puts her season at 47-13, notes it was her third comeback from a set down against Zheng this year, and records that the win makes her the 30th No. 1 in WTA history when the rankings publish Monday, ending Sabalenka's 99-week hold. At 2:08 p.m. a buyer paid 29¢ for 4,830 contracts of her title line, the highest price it touched all day. It has drifted back to 24¢ to 25¢ since.
Gauff's afternoon was worse before it was better. Andreeva won the first seven points, broke Gauff's first two service games, led 4-0 inside 12 minutes and took the set 6-2 in 26. Gauff's title contract, 22¢ at 2 p.m., printed 15¢ before 2:30 p.m. and 11¢ by 2:55 p.m., where one buyer took 6,598 contracts. It stayed between 8¢ and 20¢ through an 80-minute second set in which Gauff served for it at 5-3 and double-faulted three times in the set. Then came the tiebreak, the 39-shot rally that got her to 5-3 in it, the two match points, the sell sweep to 2¢ at 4:03:48 p.m. (the tape records prices, not points, but three bursts of selling between 4:03:48 and 4:06:24 p.m., each answered by buying, line up with a tiebreak in which Andreeva twice reached match point) and the reversal: buys at 7¢ within 30 seconds, 1,860 contracts at 10¢ at 4:04 p.m., 4,000 at 20¢ at 4:09 p.m. as the set was won, and the 9,821 at 29¢ at 4:33 p.m. She finished with 12 double faults and 12 unforced errors, per USA Today's count. The WTA's report has her at 11 straight wins, 20 of her last 22 since Wimbledon and 6-0 lifetime against Andreeva, three of the six from a set down.
There is a footnote on the other side of that match. At 2:23:16 p.m., in one second, buyers took 25,095 contracts of Andreeva's title line at 8¢. The last print on it, at 4:35 p.m., was 1¢.
One Win Each, And A 2-Hour-50-Minute First Meeting
The two have played twice on the WTA's count, and the record is 1-1. The first meeting was in Toronto in 2022, and Gauff won it 4-6, 7-6(8), 7-6(3) in 2 hours and 50 minutes. The second was in Toronto again, four weeks ago, and Rybakina won it 5-7, 6-2, 6-2 in 2 hours, her fourth three-set win of that tournament. The WTA's account of it is the one to hold onto: Gauff took the only break of the first set in its 12th game, and Rybakina said afterward that Gauff "served really well the first set" and then "started to feel a bit of pressure, some double faults."
That is the matchup in one sentence. Rybakina's serve is the best weapon on the tour by the ace count; the Cincinnati Open had her at 338 aces for the year on August 18, leading the tour, along with the most hardcourt wins of any player in 2026. Gauff's serve is the thing that cost her the first set against Andreeva and nearly the second, and her return and her legs are what won her the third. Thursday's semifinal is the tour's best server against a returner who just saved two match points and won the third set 6-2, on the court where Gauff won the 2023 title, and the market has spent all summer deciding what that is worth. The evidence is above; what the crowd decided is below.
What Kalshi Is Pricing, And How The Match Market Opened
On Kalshi, each woman is her own yes-or-no contract under the event ticker KXWTAMATCH-26SEP10GAURYB, paying $1.00 if she wins the semifinal and nothing if she does not, with the price in cents standing in for probability; the plain-English guide to prediction markets covers the mechanics and the guide to betting sports on Kalshi covers placing an order. One sentence on the rules, because it matters for a night match: a withdrawal before the first ball resolves the contract to a fair price under Kalshi's rules, while a retirement after a ball has been played pays the woman who advances.
The market listed at 4:55 p.m. ET Wednesday, about 10 minutes after Gauff's match ended, and its first hour was a market finding its feet. The first print on the Gauff side was one contract at 48¢ at 4:57 p.m., followed in the same second by 11 contracts at 82¢. By 5:14 p.m. she was printing 67¢, by 5:30 p.m. 56¢, and from 5:57 p.m. onward every fill has gone through at 54¢ or 55¢. The Rybakina leg opened at 53¢ at 5:17 p.m. and was 47¢ three minutes later, where it has stayed: 46¢ and 47¢ on every fill since 5:30 p.m. All but one of the 3,134 contracts traded on her side were buyers lifting the offer, and 97 percent of the 1,905 on Gauff's side were too. Almost nobody has sold either woman yet, the biggest sale on either leg being 28 contracts; the price came down on Gauff because the offers came down.
With the tape in hand, here is the board as it stood at 6:07 p.m. ET Wednesday, both semifinals beside the tournament board that gives the next section its argument.
| Market (Kalshi Event) | Contract | Bid | Ask | Contracts traded | Open interest |
|---|---|---|---|---|---|
| Gauff Vs Rybakina, Semifinal (Thu.) | Coco Gauff wins | 53¢ | 54¢ | 1,905 | 1,876 |
| Gauff Vs Rybakina, Semifinal (Thu.) | Elena Rybakina wins | 46¢ | 47¢ | 3,134 | 3,134 |
| Sabalenka Vs Pegula, Semifinal (Thu.) | Aryna Sabalenka wins | 64¢ | 65¢ | 43,287 | 40,469 |
| Sabalenka Vs Pegula, Semifinal (Thu.) | Jessica Pegula wins | 35¢ | 37¢ | 12,848 | 11,908 |
| US Open Women's Champion (KXWTA-26USO) | Aryna Sabalenka | 37¢ | 39¢ | 571,421 | 349,055 |
| US Open Women's Champion (KXWTA-26USO) | Coco Gauff | 27¢ | 28¢ | 858,103 | 550,907 |
| US Open Women's Champion (KXWTA-26USO) | Elena Rybakina | 24¢ | 25¢ | 484,208 | 300,355 |
| US Open Women's Champion (KXWTA-26USO) | Jessica Pegula | 12¢ | 13¢ | 410,122 | 312,776 |
Bid and ask are the live quotes on each yes contract as of September 9, 2026, at 6:07 p.m. ET; contracts traded and open interest are lifetime for each contract, all from one pull of Kalshi's public market data. The four title bids sum to 100; the four asks sum to 105, which is the spread. Both semifinals are in Thursday's evening session on Arthur Ashe Stadium, which opens at 7 p.m. ET; the order of the two matches had not been posted when this was written.
More live US Open boards: US Open men's title board (Alexander Zverev 43¢ bid, Ben Shelton 37¢) · US Open women's title board (Sabalenka 37¢ bid, 39¢ ask)
The row to sit with is Gauff's title line, because of where it was 24 hours earlier. It closed Tuesday night at 23¢, printed 22¢ at 2 p.m. Wednesday with a quarterfinal, a semifinal and a final still to play, and is 27¢ to 28¢ now with two of those three matches left. Rybakina's line did the same thing, 20¢ to 24¢ or 25¢, for the same reason. Winning a round is worth about five cents on this board, and losing a set in the round is worth about half your price for as long as the set lasts.
Pricing The Semifinal Off The Title Board
Divide the title price by the match price and you get the market's estimate of each woman's chance of winning the final, given that she gets there. On midpoints: Gauff is 27.5 over 53.5, about 51 percent. Rybakina is 24.5 over 46.5, about 53 percent. Sabalenka is 38 over 64.5, about 59 percent, and Pegula is 12.5 over 36, about 35 percent. On the asks a buyer actually pays, Gauff's number is 52 and Rybakina's is 53.
Two things fall out of those four numbers. The first is that the board thinks Thursday's two semifinals are worth the same trophy. Add the two title midpoints on this half of the draw and you get 52; add Sabalenka and Pegula and you get 50.5, and a point and a half of difference is inside a board whose asks sum to 105. Sabalenka is the single favorite, at 37¢ to 39¢, but the pair of Gauff and Rybakina is priced as a coin flip against the pair of Sabalenka and Pegula to produce the champion. The second is that the board rates Rybakina a slightly better bet than Gauff to win a final she reaches, 53 to 51, while rating Gauff the better bet to reach it, 53 to 47. The market is not confused about that. It is pricing two different finals.
Here is why the two numbers are consistent rather than contradictory. Whoever wins Gauff-Rybakina plays Sabalenka about 64 percent of the time and Pegula about 36 percent, on the other match's prices. Sabalenka leads Gauff 7-6 and won their most recent meeting, the Miami final, 6-4, 6-3, though Gauff owns the two that mattered most, the 2023 US Open final on this court and the 2025 French Open final. Gauff and Pegula are 5-5, and Gauff has won four of the last five, the latest of them the Cincinnati final three weeks ago, 6-2, 6-4. Back the board's number out: weight the final opponent 64 and 36, put Gauff at 60 percent against Pegula on that recent record, and her 51 percent implies about 46 against Sabalenka, which is what a 6-7 series with the last one lost looks like. Rybakina's version of the same sum is 8-11 against Sabalenka, but she won the Australian Open final in January from 3-0 down in the third set, per the WTA's report on her quarterfinal, and she leads Pegula 6-3 with five of the last six, including all three meetings this year. Put Rybakina at 62 against Pegula and her 53 percent implies about 48 against Sabalenka, a coin flip against a woman she has beaten in two of their last four. Two equations cannot pin four numbers, so the most this proves is that the board's implied finals are defensible against the records, and that the two semifinals it produces carry matching prices.
So the gap a reader wants, the one where the title contract is cheap relative to the match contract or the other way round, is not on this board. In payout terms, a 54-cent contract turns $54 into $100 if Gauff wins Thursday, and a 28-cent title contract turns $28 into $100 if she wins twice; the title contract is a two-match parlay priced at 54 times 52, and 54 times 52 is 28. A reader who believes Gauff's chance in the final is better than 52 percent should be looking at the title contract, and a reader who believes her chance on Thursday is better than 54 percent should be looking at the match contract. The parlay is priced fairly against its own legs. Whether the legs are right is a different question, and I have not found a sportsbook that has weighed in on it yet.
A Worked Example: What 54¢ Buys, After The Fee
With the exchange's cut included: 100 Gauff contracts at 54¢ cost $54 and pay $100 if she wins Thursday, and Kalshi's published taker fee, 0.07 times contracts times price times one minus price, rounded up to the next cent, takes $1.74 on that order. The fee pushes the break-even from 54 percent to 55.7 percent. The same 100 contracts of Rybakina at 47¢ cost $47 and pay the same $100, with a $1.75 fee and a break-even of 48.8 percent. Gauff's title contract at 28¢ costs $28 for the same $100, with a $1.42 fee and a break-even of 29.4 percent; Rybakina's at 25¢ costs $25, with a $1.32 fee and a break-even of 26.3 percent. Re-run that arithmetic at whatever price is on the screen when you read this; the numbers will have moved, and this page will not.
The one venue that disagrees with Kalshi is another exchange, not a sportsbook. Covers' semifinal picks page quotes DraftKings Predictions at 60 percent on Gauff and 36 to 37 percent on Pegula, and its handicapper recommends Gauff up to 65. The page is stamped 4:33 p.m. ET Wednesday, before Gauff had closed out her quarterfinal, so treat the six points between that quote and Kalshi's 54 as a stale quote rather than a live gap; the Pegula number sits inside Kalshi's 35¢ to 37¢. It is the only disagreement between venues I found. I did not find a posted sportsbook moneyline on the match when this was written; the live odds screen is where to shop the number across every major book against Kalshi's 54 once one is posted.
The Order Book Is Where 54¢ Lives
A market an hour old with 5,000 contracts traded should be easy to move, and this one is not, because Kalshi publishes the resting orders and they are enormous on one side. At 6:07 p.m. ET the offers on the Gauff leg read 101,724 contracts at 54¢ and 49,934 at 55¢: the 101,724 I promised at the top, all of them Gauff for sale at the ask, 53 times what her contract has traded in its life. Getting her price to 56¢ means buying every one of them. Underneath her the bids are thin, eight contracts at 53¢ and 820 at 51¢, so a seller could knock her down a couple of cents with a few hundred contracts and a buyer could not lift her at all. The Rybakina leg is thin on both sides: 3,087 offered at 47¢ and 1,368 at 48¢, with 30 contracts bid at 46¢ and 1,885 at 41¢, plus a 122,222-contract bid sitting at 9¢ that fills only if the match gets away from her while it is being played, the kind of price Gauff's title line printed on Wednesday afternoon.
Size like that on a market this new is one-sided inventory, not a quoted spread: somebody is willing to sell Gauff all day at 54¢ and has shown no interest in buying her back, since the bid under her is eight contracts. The 101,724 at 54¢ is that seller saying Gauff is worth 54 and no more, and the buyers who lifted 3,134 contracts of Rybakina at 46¢ and 47¢ are, so far, the only people who have disagreed. If there is an edge on Gauff at 54, taking it means buying into that wall, and the wall has shown its size.
What 54¢ Has To Be Wrong About
The 54 cents may well be right. What follows is what it would need to be wrong about, in both directions, priced off Wednesday's tape rather than argued from the records above.
The case for Rybakina at 47¢ is that the thing that beat Gauff in Toronto showed up again on Wednesday, and the market has already shown what it charges for it. Two early breaks took Gauff's title line from 22¢ to 11¢ inside an hour, and a match contract at 54¢ has further to fall on an early break than a title contract at 22¢ did. Rybakina is the woman on tour least likely to give those breaks back: a server who led the tour in aces in mid-August gives a returner very few looks at a second serve, and she has now come from a set down to win three matches in a row that mattered, Toronto, Melbourne and Wednesday.
The case for Gauff at 54¢ is that the same tape prices the other side too. The market that sold her to 2¢ paid 29¢ for 9,821 contracts half an hour later, and buyers lifting offers made up 71 percent of the 144,097 contracts that traded on her title line in the last 24 hours; that is a crowd that has decided the comeback is her pattern, not her exception, and a Cincinnati title plus a 2023 trophy on this court are the evidence it is leaning on. She also won the only long match these two have played, in 2022. The counter is that the 2022 match was two tiebreaks that could have gone either way and the 2026 match was not close after the first set.
If I had to name the one thing I would watch, it is Gauff's second serve in the first four games, because Toronto turned when her serve tightened after a first set she had won, and Wednesday's first set went 4-0 the other way before her serve settled. The market's 54¢ already assumes the serve holds well enough for the return to win the match. It is not priced for another 4-0 start.
More on this: Zheng Vs Rybakina Odds: Three Straight First Sets, Two Losses Anyway · Gauff vs Pegula Odds: Did Kalshi Price The Final Right? · Barcelona Vs Feyenoord Odds: Does The Favorite Survive The Fee? · The Longest Shot On This Board Might Be The Best-Kept Secret · MAC Championship Odds: Ohio Never Has To Play The Favorite
Where The Real Question Sits
So the thesis holds. The distance between the title contract and the match contract is a final against Sabalenka or Pegula, and the board prices that final about where the records say it should be, for Gauff and for Rybakina alike; the two semifinals on this exchange sit within a point and a half of each other, so there is no arbitrage between the match line and the title line, and none between the two halves of the draw. The only number on this page worth arguing with is 54¢, and what moves it before the first ball is not tennis: the order of play, which decides whether this match starts at 7 p.m. or closer to 9, any weather that pushes it later, and a practice-court injury note. What moves it after the first ball is the first four games, and Wednesday's tape has already shown the size of that move in both directions. On Thursday the board will do to one of these two women what it did to Gauff on Wednesday, and at 54¢ it is not sure which.
If pricing a single semifinal is not your game, OddsShopper's free expert picks cover the tournament the conventional way. For the wider board, the US Open odds comparison across Kalshi, Polymarket and the sportsbooks maps the whole field, the page on Rybakina's quarterfinal has the tape on how she got here, and the daily Kalshi picks hub grades its calls in public.
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These are market prices and my own reading of them, not model estimates, not predictions of fact and not financial advice. Kalshi is a CFTC-regulated event-contract exchange (18+; availability varies by state). Prices quoted are the snapshot named above and will have moved by the time you read this.
