US Open, Qualification WTA: Katie Swan vs Mary Stoiana

Platform comparison

Market context

Katie Swan’s US Open qualifying tie with Mary Stoiana is priced with Stoiana as the clear favourite across venues, while the contract’s current 0% YES implies the market is either misaligned or reflecting a stale, one-sided view. Sportsbooks have Stoiana around 1.36–1.41 and Swan around 2.77–2.87, which translates to roughly a mid-60s implied win chance for Stoiana after margin; tennis models and odds screens point to the same shape, with projected win probabilities clustered around 64–67% for Stoiana[1][2][3].

The historical frame matters because both players arrived in decent form rather than as unknown qualifiers. Stoiana, seeded 17th, had already advanced past Harmony Tan in straight sets, while Swan had beaten Katrina Scott 6-3, 6-1 to reach this round, so a live market near zero for Swan is much lower than the broader bookmaker and model consensus[4][5][6]. Comparable matches at this stage usually move on seeding, recent straight-set wins, and whether the draw is progressing on schedule rather than on headline name value alone.

Traders should watch the official order of play, court assignment, and any delay from weather or backlog in New York, because the market settles only if the match is actually completed or otherwise determined before the 14-day backstop. The BBC schedule listed the contest as an estimated evening start, and the USTA/US Open qualifying coverage showed both players were already into round two, with Stoiana carrying seeded status and Swan entering unseeded[5][6][10]. If the match is postponed, interrupted, or moved late in the session, that affects both pricing and the route to the 50-50 fallback outcome.

Sources: 1 · 2 · 3 · 4 · 5

Methodology

We track US Open, Qualification WTA: Katie Swan vs Mary Stoiana across the five venues with material prediction-market liquidity. The probability shown is the live Polymarket mid; the comparison rows summarise how each venue treats the underlying contract — fees, KYC thresholds, settlement currency, deposit options. The highlighted row marks the cheapest route into Polymarket's order book.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

UK Frequently Asked Questions

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Best Prediction Markets UK. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

What are the best prediction markets for UK traders in 2026?

For UKGC-regulated markets: Betfair Exchange (sports, politics) and Smarkets (sports, politics, lower commission than Betfair). For broader global event prediction: Polymarket (deepest liquidity, 0% fee, USDC settlement). For US-regulated option: Kalshi (USD, limited UK payment options). Most serious UK traders use a combination of Betfair for regulated sports and Polymarket for broader markets.