There are reasons to believe that each one will have the edge.
For Alcaraz, the most obvious reason is their head-to-head. He leads it 8-5, and before his loss at Wimbledon, he had won five in a row. While Sinner dominates on hard courts against everyone else, Alcaraz is 5-2 against him on the surface.
Sinner, put simply, brings out the best in Alcaraz. The Spaniard has the highest ceiling of anyone when it comes to speed, power, and athleticism—few, if any, players in the game’s history have been as electric. But he doesn’t typically bring a total focus, or stay at his best, for the length of a match. Against Sinner, though, he tends to dial in and find a way to reach his top level by the end. We saw it last fall in Beijing, and at Roland Garros, when he ran away with the final-set tiebreakers.
Sinner has the edge in overall consistency from one match and tournament to the next, and his ability—Roland Garros notwithstanding—to win the big ones. That’s why he has been No. 1 for the last 62 weeks. He’s 20-7 in finals, 17-5 on hard courts, and he’s won the last three Grand Slams played on that surface. He’s also the defending champion in Cincinnati, a tournament Alcaraz has yet to win. Sinner’s game requires precision, but he has shown an ability to sustain his best, over a longer period of time, than Alcaraz.