National Bank Open: Marta Kostyuk vs Katherine Sebov

Platform comparison

Market context

Marta Kostyuk is the clear favourite against Katherine Sebov in the National Bank Open second round, and the market’s 100% YES price is consistent with the ranking gap, surface fit and their past meeting. Kostyuk is WTA No. 11, while Sebov is a wildcard around No. 231, and their only prior head-to-head was won by Kostyuk in straight sets on hard court at Chicago in 2021.[2][4][15]

For a programme or bot reading this market, the practical approach is to treat the current price as a near-locked outcome unless the event mechanics change. The official match page already records Kostyuk leading on completed statistics, with the contest shown as played on Centre Court and the scoreline in Kostyuk’s favour, which is the kind of state a resolver would use to convert a simple “winner advances” market into a settled branch rather than leaving it contingent.[3][8] In comparable WTA events, a strong favourite on outdoor hard court with a previous straight-sets win over the same opponent normally trades close to certainty once the draw and scheduling are confirmed.[1][14]

The main catalysts to watch are not tennis form but operational dependencies: whether the match starts, whether it is completed, and whether any post-draw change affects the official advancement status. Tennis listings from multiple scoreboards show the fixture as scheduled for 4 August in Toronto, and the settlement rule only reverts to 50-50 if the match is cancelled, tied, or delayed more than seven days without a winner.[3][6] Programmatically, that means the relevant signals are official completion status, any walkover or retirement notation, and whether the resolver window closes before 11 August 2026 at 18:00 UTC.

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page reviews National Bank Open: Marta Kostyuk vs Katherine Sebov across five venues. The live probability is the Polymarket mid-price, sourced directly from the on-chain Polygon order book; the comparison columns benchmark each venue on fee structure, KYC, settlement currency and payment rails. Every CTA routes to Polymarket Bot UK, which mirrors the Polymarket order book at 0% fees.

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.

FAQ

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 Polymarket Bot 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.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.