Calibrated ~0.70 probability the 2026 US Open men's champion is Sinner OR Alcaraz — locked as a clean top-2 partition, not a single-player pick. Sinner is World No.1 by ~3,500 pts on his home surface; the joint probability is anchored on market-implied data.
Sinner World No.1 by ~3,500 pts (ATP, Jun 2026). Market-implied individual probabilities (US Open outright odds) sum to ~0.65–0.72 for the top-2 pair after vig removal. We calibrate to ~0.70.
We lock a top-2 partition: P(2026 US Open men's singles champion ∈ {Jannik Sinner, Carlos Alcaraz}) ≈ 0.70. We do not pick between them — that is a coin-flip inside the pair and a different question. The remaining ~0.30 covers the rest of the 128-draw field.
Sinner is World No.1 by ~3,500 pts over Alcaraz (ATP live rankings, Jun 2026) and defending US Open champion. US Open hard court is his strongest surface. Alcaraz is No.2 but has carried an injury returning from clay. Betting-market implied probabilities (vig-removed) for the pair sum to ~0.65–0.72 depending on the book; we calibrate our joint lock to ~0.70. Resolves against the official men's singles champion declared by usopen.org on or before Sep 13 2026.
RAOSCAFF locks P-23 on 2026-06-21, before the 2026 US Open (Aug 25–Sep 13). Resolves binary: YES if the champion is Sinner or Alcaraz, NO otherwise.
One binary partition, one official source — no spin possible at resolution.