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.
Goran Ivanisevic's son made an adorable cameo, too, in the Turin clip where candy, doubles partners and Santa Claus came up.