The Serbian can still win the tournament, which remains in the roudn-robin stage.
The 22-year-old Italian won a three-hour marathon 7-5 6-7 (5) 7-6 (2).
Novak Djokovic has congratulated Jannik Sinner, after the Italian ended the world No.1's 19-match winning streak last night in Turin.
How does Novak Djokovic stay motivated? Plus, another perspective on Coco Gauff's U.S. Open speech.