Novak Djokovic's winning streak was halted at 19 matches as home favourite Jannik Sinner was roared to a superb 7-5, 6-7(5), 7-6(2) victory by a partisan ATP Finals crowd in Turin on Tuesday.
The earlier Green Group clash was a non-event as Stefanos Tsitsipas retired injured against Holger Rune, but the evening ticket holders were treated to a match of jaw-dropping quality.
After more than three hours of an absorbing toe-to-toe battle in which neither player took a backward step it was world number four Sinner who seized his chance in the deciding set tie-break finally to crack Djokovic's resistance.
Sinner, 14 years Djokovic's junior, wrapped up his first career win against the 24-time Grand Slam champion with a smash - prompting deafening roars from a sell-out crowd who produced an electrifying atmosphere throughout.
He moves top of the group but is not yet guaranteed a semifinal spot ahead of his final round-robin match against Rune on Thursday when Djokovic, who will end the year as No. 1 for a record-stretching eighth time, plays alternate Hubert Hurkacz.
Sinner, Djokovic and Rune could all still end up with two wins each in a group that is too close to call.
Sinner has enjoyed his best year on Tour and now has 59 match wins in 2023, but none would have given him more pleasure than taking down six-time ATP Finals champion Djokovic.
The Italian moved 2-0 ahead in the deciding tiebreak with a rocket of a forehand return and then gave himself daylight with another forehand winner to make it 3-0.
Once he moved 5-0 ahead not even Djokovic's powers of recovery could spare the Serb his first loss since defeat by Carlos Alcaraz in the Wimbledon final.
"It's a sort of process, I feel that I'm more confident in certain moments," Sinner told Amazon Prime.
"In the second set tie-break he played a bit better than me but I think I was brave in the big moments. We were both serving very well, I think we both played really, really well."
Reigning champion Djokovic did not do much wrong as he battled Sinner and the crowd - regularly jesting with the fans who jeered him throughout the duel.
Sinner made his move at 5-5 in the opening set as Djokovic briefly lost focus after leading 40-0 on serve.
A double-fault at deuce was cheered by the crowd and Sinner then converted the break with two punishing backhands. Sinner then held serve to love to move in front.
Djokovic raised his level a notch in the second set and dropped only five points on his serve before the tiebreak which he edged to extend the contest into a decider.
Sinner broke for a 4-2 lead but Djokovic broke back immediately. When Sinner then moved 6-5 ahead on serve, Djokovic sat in his chair and conducted the jeers before a calm service hold sent an epic tussle into a shoot-out.
The smart money would have been on Djokovic to make a 20-match unbeaten streak for the ninth time in his career, but Sinner had other ideas as he chalked up one of the biggest wins of a career that now seems on the verge of lift-off.
"To beat the world No. 1, who has 24 Grand Slams, that means a lot, it's one of the top (wins)," he said.
The match statistics summed up the quality on offer with a combined 83 winners and only 25 unforced errors in a contest lasting three hours and nine minutes.
Asked about the raucous crowd, Djokovic said: "I expected it, he's the only Italian here and he's playing in Italy. There's a lot of hype and he's in great form, so it's normal."
