The event is definitively resolved as No. Iga Świątek lost to Qinwen Zheng in the US Open round of 16 on September 7, 2026, before reaching the semifinals. Although prior forecasts assigned a 40%–44% chance based on her ranking, form, experience, and favorable expectations, the completed match result is conclusive evidence. Since an eliminated player cannot later reach the semifinals, the likelihood of the event occurring is effectively zero and the posterior probability is 0%.
Prior: The recent forecast history was 41%, 44%, and 40%, implying a baseline near 41% for Świątek reaching the semifinals given her draw, season form, and likely quarterfinal opposition. I use approximately 41% as the starting point.
Update 1 — reaching the round of 16: Her three straight-set wins, 13–1 recent record, Toronto title, and strong US Open history are positive evidence. This raises the estimate relative to a pre-tournament baseline, although much of this evidence was already reflected in the prior forecasts.
Update 2 — round-of-16 matchup: Świątek is favored against Zheng, with an estimated 75%–82% chance of winning. The favorable head-to-head record and Zheng’s four consecutive three-set matches support Świątek, while Zheng’s momentum and comeback win over Keys prevent a higher estimate.
Update 3 — likely quarterfinal: The key obstacle is a probable matchup with Rybakina. Świątek’s estimated chance in that match is only about 45%–52%. A possible Osaka quarterfinal would be somewhat more favorable, but Rybakina is the likelier opponent and has advanced without dropping a set.
Update 4 — current performance and draw: The elimination of Madison Keys is modestly favorable, but Świątek’s two-tiebreak win over Bouzková was physically demanding and less dominant than her earlier victories. Rybakina’s strong form offsets much of the benefit from Keys’ elimination.
Combining an approximately 78% chance of beating Zheng with a roughly 50%–52% weighted chance of winning the quarterfinal produces a result near 39%–41%. My final estimate is 40%. The event is plausible, but winning the likely quarterfinal remains approximately a coin flip.
Prior to the tournament, a reasonable baseline was approximately 30%–35%, reflecting Świątek’s No. 8 seed, inconsistent 2026 season, recent US Open quarterfinal exits, and a difficult projected quarter containing Rybakina, Osaka, Keys, and Zheng. Her two efficient straight-set wins over Wang Xiyu and Nadia Podoroska provide positive evidence: she has looked healthy, has not dropped a set, has reportedly not faced a break point, and has conserved energy. I would apply a moderate upward update to roughly 40%–42%, rather than a large one, because the opponents so far were weaker tests. The Toronto title and recent straight-set win over Rybakina also improve the estimate, since they show strong current hard-court form and evidence she can handle the likely quarterfinal threat. However, the probability remains below 50% because she still likely needs to defeat Bouzková, then a substantially stronger round-of-16 opponent, and finally win a difficult quarterfinal—potentially against a healthy Rybakina. Approximate conditional probabilities of 85% to beat Bouzková, 70%–75% in the round of 16, and 50%–55% in the quarterfinal imply a combined probability near 30%–35%, but favorable uncertainty about the draw and Rybakina’s fitness supports a somewhat higher overall estimate. Balancing these factors and the prior forecasts of 41% and 44%, my current estimate is 40%.
Starting from the previous forecast of 41%, I treat that as the prior incorporating her ranking, draw position, historical Grand Slam performance, and the uncertainty of winning four additional matches to reach the semifinals. The first-round victory was highly convincing—straight sets, no break points faced, and strong first-serve performance—so it provides a modest positive update (likelihood ratio above 1), raising the estimate by roughly 3 percentage points. Her recent hard-court form is also favorable: winning Toronto and reaching the Cincinnati semifinal supports good playing conditions and confidence, adding another modest positive update. The coaching change, decision to skip mixed doubles, and apparent energy management are smaller positives, but their predictive value is uncertain. Against this, she has exited in the US Open quarterfinals in each of the last two editions, and as the No. 8 seed she could face elite opponents in the later rounds. Those factors prevent a large increase. Overall, the evidence supports a moderate rise from 41% to 44%, but reaching the semifinal still requires several difficult wins and remains less likely than not.
Prior: I estimate roughly a 32% baseline probability for an elite but non-top-4-seeded player to reach the US Open semifinals. Reaching the final four requires winning several matches, and the women’s field is deep, so even a strong contender is more likely than not to fall before the semifinals.
Update for historical US Open performance: Swiatek won the 2022 title and made the quarterfinals in 2024 and 2025, but also exited in the round of 16 in 2023. This supports a modest positive adjustment, but her recent quarterfinal exits also indicate that the QF-to-SF step has been difficult. Net effect: approximately neutral to slightly positive.
Update for 2026 hard-court form: Winning Toronto and reaching the Cincinnati semifinals are strong evidence of current surface suitability and match rhythm. I apply a positive likelihood adjustment, increasing the estimate from about 32% to approximately 40%.
Update for ranking and seeding: A No. 8 ranking should provide a favorable draw relative to an unseeded player and reduce the chance of facing another elite opponent early. This raises the estimate modestly, to about 42%.
Update for winning the opening match: A comfortable first-round victory confirms fitness and immediate form, but it is weak evidence because Swiatek was heavily favored. I increase the estimate only slightly, to approximately 43%.
Offsetting risks include the unknown draw, possible fatigue, elite quarterfinal opponents such as Jessica Pegula, and Swiatek’s recent difficulty converting US Open quarterfinal appearances into semifinal berths. After accounting for those risks, my final forecast is 41%.