Sinner, 22, was cheered on by a partisan home crowd as he won the group match 7-5 6-7 (5-7) 7-6 (7-2) – his first career victory against the 24-time grand slam singles champion.
“That was a really high-level match,” the world No. 4 Italian said after the win, per ATP.
“I think I was really brave and intelligent in important moments, especially the third set.
“I felt like we were both serving really well, so there were not so many exchanges, but when the exchanges were played, we both played really well.”
Sinner is part of a new generation of players threatening to take over from the likes of Djokovic, who is 14 years his senior.
There were times during Tuesday’s match when the Serbian superstar, arguably the greatest player of all time, could only applaud what his opponent was producing on the other side of the net.
Sinner’s booming ground strokes caused consistent problems for Djokovic, who seemed to spend much of the match battling to stay in the contest.
Even when Djokovic used his experience to turn up the pressure, Sinner seemed to find an extra gear – ending the match with an overhead smash after racing ahead in the final set tie-break.
‘Courageous’ Sinner prevails
The pair shared a warm embrace at the net after the match, as the Italian crowd applauded its home hero.
“I think the main difference is that, in the important points, he was going for it, he was more courageous,” Djokovic said, per ATP.
“He deserved to win because, in important moments, I wasn’t aggressive enough, I wasn’t decisive enough. I gave him the opportunity to take the control over the points.
“You have to just congratulate him. He just played a fantastic match. That’s what I told him at the net. I think in the most important moments, he played his best game and he absolutely deserved to win.”
It was Sinner’s second win at this year’s ATP Finals – the end-of-season competition for men’s tennis.
The Italian has not yet secured qualification into the semifinals, though, and will play Denmark’s Holger Rune on Thursday as he seeks to finish the group stage unbeaten.
Meanwhile, Djokovic will play Poland’s Hubert Hurkacz, needing a win to assure progression through the group.
Hurkacz came in as an alternate to Stefanos Tsitsipas after the Greek star injured his back earlier this week.
The ATP Finals sees the eight top-ranked men battle it out to become the end-of-season champion. Players are split into two round-robin groups, with the winner and runner-up of each progressing to the semifinals.
Djokovic is the reigning champion after beating Rune in the 2022 final. The 36-year-old has won six ATP Finals titles during his career, including four consecutive wins between 2012-2015.