Cincinnati Open: Marta Kostyuk vs Sloane Stephens

Platform comparison

Market context

The Cincinnati Open match between Ukrainian left-hander Marta Kostyuk and American former US Open champion Sloane Stephens is scheduled for 18 August 2026. The 100% crowd probability reflects confidence the match will proceed as scheduled, though the settlement window extends to 25 August to account for potential delays or rescheduling within the tournament's standard seven-day window.

Kostyuk, ranked in the top 20, has demonstrated consistent hard-court performance over recent seasons, whilst Stephens has experienced variable form since her 2017 peak. Their head-to-head record and recent tournament trajectories matter less than fixture stability at Cincinnati, where weather disruptions and scheduling conflicts are routine. The crowd's certainty suggests no reported injury concerns or withdrawal signals for either player as of the market's opening, a baseline assumption that typically holds through the week before a major hard-court event.

Traders should monitor the ATP and WTA official draw confirmations and any player injury updates through 17 August. Cincinnati's outdoor hard courts occasionally face rain delays; the settlement terms allow for rescheduling within seven days without triggering a 50-50 resolution. Stephens' recent tournament entries and Kostyuk's injury status are the primary catalysts to track. Any withdrawal announcement would immediately shift probability, though the current reading suggests both players are fit and committed to competing.

Methodology

This page is a comparison snapshot: one live quote, four reference venues with their key attributes, and a single execution path — every trade button routes to Prediction Today, which mirrors the Polymarket order book directly.

Resolution & payout

Settlement runs on-chain. Polymarket's contract logic separates YES and NO shares as conditional tokens; at resolution the winning share lifts to $1.00 and the losing one to $0. The outcome input comes from the UMA Optimistic Oracle, which secures against bad resolution with a bond + dispute window.

Once finalised, the smart contract pays USDC to the holders' wallets within minutes — no withdrawal fees beyond Polygon network gas. Kalshi settles in USD via CFTC clearance, Betfair in account currency net of commission, Manifold in play-money mana with no cash-out.

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 Prediction Today. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

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.

Is Polymarket legal in the UK?

Polymarket is accessible to UK traders but is not UKGC-licensed. It operates under US CFTC jurisdiction. UK residents face no domestic legal prohibition on using it, but UKGC consumer protections do not apply. For UKGC-regulated alternatives, Betfair Exchange and Smarkets offer similar prediction-style markets.

Do I pay tax on prediction market profits in the UK?

UKGC-licensed platform profits (Betfair, Smarkets) are typically tax-free gambling winnings for UK individuals. Polymarket profits involve USDC crypto transactions, which HMRC treats as Capital Gains Tax events. Keep full transaction records and report via Self Assessment if gains exceed £3,000 per tax year.

How do I deposit on Polymarket from the UK?

UK traders typically fund Polymarket via Coinbase UK, Kraken or Revolut — buying USDC with GBP and transferring to a Polygon-compatible wallet (MetaMask, Coinbase Wallet). Polymarket's onboarding walks you through the bridging process. Typical GBP-to-USDC conversion costs 0.5–1%.