A Kalshi prediction market trader pocketed nearly $4 million after correctly predicting a major upset at the US Open on Tuesday.
Key Takeaways
- Shelton only had about a 25% chance of winning before the match.
- Live probabilities swung sharply as momentum shifted throughout the match.
- Kalshi took nearly $50 million in trades on the match.
Kalshi said in an X post Tuesday evening that it had processed a $888,234.14 position on Shelton to defeat Alcaraz, despite users giving the Spaniard more than a 75% chance of winning. The position had a potential payout of $3,753,655.90 if Shelton won.
JUST IN: A Kalshi user has an $888,234.14 position on Ben Shelton to defeat Carlos Alcaraz tonight.
— Kalshi Sports (@KalshiSports) September 9, 2026
Potential payout of $3,753,655.90. pic.twitter.com/2RTh1OdMXP
Alcaraz entered the match ranked No. 3 in the world, while Shelton was No. 9 and had never beaten the Spaniard in three head-to-head matchups.
“Buddy wasted $888,234.14,” one user replied to Kalshi’s post confirming the trade.
“Man makes charitable donation to Kalshi,” another wrote.
The match didn’t end until 3:33 a.m. in New York, making it the latest finish in US Open history. Shelton had to come from behind numerous times, including from a 5-3 deficit in the first-to-10 deciding-set tiebreaker at the end of the four-and-a-half-hour battle. The final score was 6-7(5), 6-1, 6-3, 1-6, 7-6(10-7).
BEN SHELTON DID IT!
— Kalshi Sports (@KalshiSports) September 9, 2026
Shelton wins a 5-Set classic against Carlos Alcaraz for the biggest win of his career and a spot in the US Open Semifinals!pic.twitter.com/qq8Q8FaquL
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Kalshi sees shift in US Open prediction markets
Kalshi’s US Open prediction markets saw heavy activity before and during the quarterfinals. Shelton had risen to a 60% chance of winning at 1:30 a.m. and later fell back to 21% during the final tiebreaker before rallying for the win.
The leading prediction operator reported $49.6 million in trades on Shelton-Alcaraz. That was the runaway leading amount for all matches in this round, including Francis Tiafoe against Alex Michelsen ($35.8 million), Karen Khachanov versus Alexander Blockx ($306,296), and Alexander Zverev against Botic van de Zandschulp ($199,182). The last two of those matches will be played Wednesday.
Tiafoe and Shelton will face each other at 3 p.m. on Friday. Kalshi prediction traders give Shelton a 72% chance of winning the match, while Tiafoe - the world’s No. 12 player - has a 28% chance. At the time of writing, $61,413 has been traded in this market.
Shelton is a heavy favorite to reach the men’s singles final but is still an underdog to win the tournament. Zverev is the leading candidate at 44%.
Notably, Zverev’s likelihood of winning spiked nine percentage points after Shelton defeated Alcaraz. Alcaraz had a 56% chance of winning the tournament when his match against Shelton began.
Kalshi extends prediction market lead
Kalshi and its prediction markets are growing at an extraordinary pace. The company announced Tuesday morning that its commodities trading volume reached $400 million in seven months, roughly half the time it took crypto.
In related prediction market news, Kalshi generated a reported $38.7 billion in trading volume during August, barely dropping despite the post-World Cup hangover. Polymarket, its top rival, suffered a larger setback, dropping about 35% to $8.4 billion - about 21.7% of Kalshi’s figure.
