Jannik Sinner’s only US Open challenge left is measuring up to himself

LONDON, ENGLAND - July 12: Jannik Sinner of Italy during his match against Alexander Zverev of Germany in the Gentlemen's Singles Final on Centre Court at the All England Lawn Tennis and Croquet Club during the Wimbledon Lawn Tennis Championships on July 12th, 2026, in London, England. (Photo by Tim Clayton/Getty Images) | Getty Images As he gears up for the 2026 US Open, what does Jannik Sinner have left to chase? I mean, the guy has already won five Grand Slams at 25 years old, he’s made…