As the US Open’s main draw got underway Sunday, Front Office Sports has learned the tournament secured a new exclusive prediction-market partner at the last minute: Kalshi.

Effective immediately, Kalshi will be the exclusive prediction-market partner of the US Open, two sources tell FOS. Terms and parameters of the deal were not clear, but the agreement was not formally locked in until after last week’s qualifying rounds had concluded, the sources say. 

The US Open, which is owned and operated by the U.S. Tennis Association, did not originally intend to have a prediction-market deal in place for this year’s tournament, the sources say; in fact, in recent weeks it held discussions with multiple platforms, but talks centered around match-integrity concerns and potential partnerships for next year and beyond.

However, new USTA CEO Craig Tiley, who joined the organization in February after more than a decade as CEO of Tennis Australia—the entity that puts on the Australian Open—played a major role in getting a deal done for this year’s tournament, sources tell FOS.

One source says that an “unusual” aspect of the agreement is that the USTA is blocking any other prediction-market platforms from advertising in the facility or on TV, including across ESPN, where the tournament is being broadcast. 

Earlier on Sunday, Kalshi published a blog post about the tournament, saying its platform has Aryna Sabalenka as the favorite to win at 23%, with Coco Gauff and Iga Świątek as close contenders. That blog post included fine print saying “Kalshi is not affiliated with the U.S. Open or WTA.” As of early Sunday afternoon, roughly $1.5 million had been placed on who will be the women’s singles winner. 

Sources say the USTA went forward with this deal even though, as of last week, it hadn’t formally approved prediction markets as a category to partner with. As of Sunday afternoon, Kalshi did not appear on the US Open’s list of official partners. No traditional sportsbooks are listed as official partners either.

The ATP Tour, which oversees the men’s professional tennis tour and rankings, has no direct role in operating the US Open, but it has separately weighed in on prediction-market regulation. In a comment letter sent to the Commodity Futures Trading Commission in April, it expressed support for the regulator’s proposed rules to govern sports-event contracts and offered “recommendations that we believe would strengthen [the proposed rules],” including a prohibition on event contracts related to player injuries and officiating decisions.

Prediction Markets Sports Spree

Despite ongoing questions about whether sports-event contracts are legal under a federal framework or must be regulated at the state level like traditional sports betting, prediction-market platforms have been jockeying for sports partnerships. Both Kalshi and Polymarket are official partners of the NHL, while Polymarket landed the overall deal with MLB and has an agreement with the Yankees that runs through the rest of this season. Kalshi has team-level deals with the NHL’s Blackhawks and MLB’s Giants, Braves, Padres, Red Sox, and Dodgers; the first prediction-market company to partner with an individual team was Novig with the Mets.

Among the major pro U.S. sports leagues, the NFL and NBA remain holdouts. The NFL will not have any prediction-market deals in place when the regular season kicks off Sept. 9, while the NBA has been in talks with multiple platforms for over a year but has not yet formally embraced the industry.

Kalshi and ESPN declined to comment. The USTA did not immediately offer comment.