Jannik Sinner ends Novak Djokovic's winning streak with thrilling victory.
Jannik Sinner claimed his first ever victory against Novak Djokovic as the Italian beat the world number one at the ATP Finals in Turin.
Sinner, 22, won a third-set tie-break to secure the group match 7-5 6-7 (5-7) 7-6 (7-2) in three hours and nine minutes.
It was 36-year-old Djokovic's first defeat since the Wimbledon men's final in July.
Earlier, Stefanos Tsitsipas withdrew from the competition through injury.
Tsitsipas, 25, retired through injury against Denmark's Holger Rune after consulting with a doctor at the changeover.
"It kills me not to be able to finish this tournament," the Greek said.
Sinner, who will play Rune in his third match, ended Djokovic's 19-match unbeaten run in front of his home crowd.
However, the 24-time Grand Slam champion can still qualify for the semi-finals by beating Polish ninth seed Hubert Hurkacz, Tsitsipas' replacement.
After Sinner took the first set, Djokovic levelled and fought back from 4-2 down in the third before being outplayed in the deciding tie-break.
"I think I was really brave and intelligent in important moments. We both played really well, it was a high-level match," world number four Sinner told Amazon Prime.
"It means a lot to me, when you win against the world number one who has 24 Grand Slams, it's the top.
"It was a tactical match and I managed to win so I'm really, really happy."
In the earlier match, Tsitsipas, the world number six, trailed Rune 2-1 in his group match and appeared to be troubled by a back problem and left the court to a few jeers from the crowd inside Pala Alpitour.
"I'm really gutted that I wasn't able to finish the match," he said. "It's a shame also not to give that spot to someone else to at least try and do something with it.
"Unfortunately, I felt terrible on the court. I did what I could do in the best possible way to be ready and fit for this match, but it didn't work out for me."
"For sure it was very unfortunate," 20-year-old Rune said. "Even in the first service game you could see he wasn't serving anywhere near full like he can, I knew something was off."