Kostyuk vs Stephens: Prediction, Odds & Probability
By Corbin King · Updated September 2, 2026
As of September 2, 2026, the prediction market implies a 88% chance of Marta Kostyuk on Kalshi.
Implied probability snapshot
Market favorite
Marta Kostyuk — market favorite
Kalshi
88%
Implied
Venue listing
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Driving coverage
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Odds — dated market snapshotas of September 2, 2026
| Outcome | Venue | Implied | Volume |
|---|---|---|---|
US Open WTA: Marta Kostyuk vs Sloane Stephens Moneyline | Polymarket | 87% | $12.7K |
US Open WTA: Marta Kostyuk vs Sloane Stephens Set 2 Winner Moneyline | Polymarket | 81% | — |
US Open WTA: Marta Kostyuk vs Sloane Stephens Set 1 Winner Moneyline | Polymarket | 79% | — |
Marta Kostyuk | Kalshi | 88% | 4.3K contracts |
US Open WTA: Marta Kostyuk vs Sloane Stephens Set Handicap +/-1.5 | Polymarket | 66% | — |
US Open WTA: Marta Kostyuk vs Sloane Stephens Set 1 O/U 8.5 | Polymarket | 58% | $8 |
US Open WTA: Marta Kostyuk vs Sloane Stephens Game Spread +/-5.5 | Polymarket | 53% | $4 |
Completed Match | Polymarket | 50% | — |
Implied probability = the venue’s YES price read as a percentage at capture. This is a dated observation; use the attributed venue link, when available, to verify later changes.
W.E.T. Score
How much signal is behind this number?
The W.E.T. Score grades the evidence, not the outcome. Five inputs — comparable volume evidence, news corroboration, cross-venue verification, market coverage, and track-record depth — scored against how much verifiable signal sits behind the headline probability.
Scored from the September 2, 2026 snapshot · 32/100 · Thin Signal
| Component | Points | What the data says |
|---|---|---|
| Liquidity | 10/30 | The event combines unlike venue-native volume units. W.E.T. does not add contracts to dollars, so this component remains at the documented neutral 10/30. |
| News corroboration | 0/20 | No wire headlines matched this event at snapshot time — the price stands alone. |
| Cross-venue verification | 8/20 | No canonical exact-match pair is available for this snapshot. Related listings are not compared, so this component remains at the documented neutral 8/20. |
| Market coverage | 12/15 | 8 tracked markets on this event — more side markets and strikes mean more angles pricing the same story. |
| Track-record depth | 2/15 | 1 attributable day tied to one venue, favorite, and listing key. Legacy unkeyed points earn no depth credit. |
The W.E.T. Score measures evidence quality — comparable volume, verified agreement, coverage, and corroboration behind a price. Venue-reported contract counts are never treated as dollars. It is not a prediction of the outcome, not a rating of whether the favorite wins, and not trading advice: a Thin Signal market can still resolve YES, and a Prime Signal market can still resolve NO. Full weighting and worked examples in the methodology.
The desk layer
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Key takeaways
- One tracked Kalshi listing prices Marta Kostyuk at 88% implied probability.
- Venue volume remains separate because this event combines dollar volume and contract counts.
- This is implied probability from venue prices observed as of Wednesday, September 2 — not a tip or a forecast of the result.
Prediction markets make Marta Kostyuk the favorite in Kostyuk vs Stephens, pricing the outcome at 88% implied probability on Kalshi as of Wednesday, September 2. Related venue listings are shown separately; W.E.T. has not confirmed them as the same outcome for a numerical comparison. Here is the dated market read.
What the prediction market is pricing
Across 18 tracked market rows, Venue volume remains separate because this event combines dollar volume and contract counts. Marta Kostyuk leads at 88%, a strong favorite, ahead of Sloane Stephens at 14%. Related venue listings remain separate until the canonical graph confirms outcome and settlement-basis equivalence.
How sports markets like this settle
Sports markets resolve on the game itself — a final result, a score line, a player total. The implied probability is what traders are collectively paying for each outcome, repriced live as lineups, injuries, and in-game events land.
What could move it
Team news and injuries, roster and lineup changes, weather for outdoor games, and the way the line drifts from open to close as sharp money arrives are what move these prices. Because real money is on the line, the price reprices quickly around these catalysts — which is what makes 88% a live estimate rather than a settled call.
Reading the number
A contract at 88¢ implies a 88% chance the market resolves YES: a dated venue observation of implied probability, not a forecast or a tip from W.E.T. The figure above is a snapshot as of Wednesday, September 2; odds move continuously, so open Kalshi for the current line.
Track this and every Sports market on the live Event Dashboard, or browse all Sports odds & probability. For the mechanics, see the prediction markets guide.
Frequently asked questions
Who will win Kostyuk vs Stephens?
As of Wednesday, September 2, Kalshi price Marta Kostyuk at 88% implied probability. Implied probability is the market's YES price read as a percentage; it moves continuously, so check the live venue for the current line.
What is the probability of Marta Kostyuk?
The prediction market implies a 88% chance of Marta Kostyuk on Kalshi as of Wednesday, September 2 — that is what traders are collectively pricing, not a forecast from W.E.T.
What is the difference between the Kalshi and Polymarket odds?
Both venues carry related listings. W.E.T. publishes them separately until canonical identity and settlement-basis checks confirm that a numerical comparison is valid.
Are these odds a prediction of the outcome?
No. These are dated venue observations shown as implied probability — a record of what traders were collectively paying at the stated time, not advice or a forecast from W.E.T. Prediction market trading involves risk of loss; always verify current prices and rules on the venue.
Markets & sources
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Preview
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<p style="font:13px/1.4 sans-serif;margin:6px 0 0">Odds via <a href="https://www.worldeventtrading.com/predictions/kostyuk-vs-stephens-odds-2026-09-09?utm_source=wet-embed&utm_medium=attribution&utm_content=kostyuk-vs-stephens-odds-2026-09-09">World Event Trading</a></p>W.E.T. is an information and data service — not a gambling operator. We take no bets and hold no funds. Nothing here is financial, legal, betting, or tax advice, or a prediction of the outcome, and prediction market trading involves risk of loss. Prediction markets may be restricted or illegal in your jurisdiction — check local law. 18+. See how we source and compute these odds in our methodology, or read the full disclaimer.
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