A nerveless Stefanos Tsitsipas came from a set down to beat Dominic Thiem on a thrilling final-set tie-break and become the youngest winner of the ATP Finals in 18 years.
The Greek, 21, won 6-7 (6-8) 6-2 7-6 (7-4) to claim the biggest title of his fledgling career.
Thiem, 26, who edged a tight first set, fell away in the second and the start of the third before fighting back.
But Tsitsipas forced a tie-break, and then won it, at a raucous O2 Arena.
He collapsed to the floor after Thiem put a forehand return out on the first match point before the pair embraced at the end of a two-hour 35-minute battle, with the Greek kissing his beaten opponent on the shoulder.
Tsitsipas is the youngest winner of the season-ending championships since Australia’s Lleyton Hewitt in 2001 and claims prize money of just over £2m.
“It’s been a roller-coaster. Holding this trophy is amazing,” the world number six said.
“It is a dream come true and the best way to end this match.
“I did get nervous at some points in the match but I managed to forget about how I felt and I had momentum in the tie-break, which was really tight.
“My fighting spirit and me constantly trying to push myself to do better got me there in the end.”
Tsitsipas takes advantage of Thiem’s lapses
Tsitsipas broke the hearts of many in London on Saturday in knocking out six-time champion Roger Federer and a day later backed that up with a performance that showed why he is regarded as a player who will be one of those at the top of the game when the Swiss and the other current greats move on.
He had been the better player in the opening set only for Thiem to take it in on a tie-break but when the Austrian’s level dropped he was there to take advantage.
He did not panic and hit just one unforced error in the second set compared to eight from his opponent – going 4-0 ahead in 14 minutes and wrapping up the set soon after.
Tsitsipas’ entertaining game – full of powerful forehands, laser-like serving and a stunning single-handed backhand – improved further at the start of the third set and he looked set to power on to the title.
But Thiem, a two-time Grand Slam finalist, broke back and levelled as his crunching groundstrokes returned.
In the tie-break Tsitsipas saw a 4-1 lead become 4-4 before he held his nerve to take the match with three straight points.
More to follow.