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.