Stefanos Tsitsipas battled back from a set down to defeat Austrian Dominic Thiem 6-7 (6/8), 6-2, 7-6 (7/4) to claim a maiden ATP Finals title.
Tsitsipas emphatically claimed a third title of the season and comes a year after he was crowned champion at the Next Gen ATP Finals in Milan.
He has charmed the crowd at London's O2 Arena all week and earned his place in the final with an impressive straight-sets win against six-time champion Roger Federer.
Defeat is a bitter disappointment for Thiem, who won five titles this year, including the Indian Wells Masters, and reached a second successive French Open final. There was little to choose between sixth-seed Tsitsipas and Austria's world number five Thiem in the first set as a total of five break points went begging.
Both players had impressed on serve during the tournament, with Tsitsipas's figures especially striking -- 44 service games won out of 47 leading into the final.
The Greek held with ease and then produced a forehand down the line to break again, with Thiem threatening to unravel. He had won 12 out of 14 points in the set.
Thiem, who like Tsitsipas plays with a single-handed backhand, held to love in the fifth game to stem the tide but he could make little impression on his opponent's serve.
He netted with a backhand to hand Tsitsipas the set 6-2.
Thiem was rocking again at the start of the deciding set. He saved two break points to hold in the first game but looked increasingly shaky and was broken to trail 1-2.
The Greek was now well on top but out of nowhere, Thiem was level at 3-3 after Tsitsipas put a backhand wide. The Austrian was now playing with much more confidence, unleashing a series of blistering winners and edged ahead 5-4 after another service hold.
The set went to a tie-break. Tsitsipas forged ahead but Thiem pegged him back until the Greek was gifted another mini-break when the Austrian went long. Tsitsipas made no mistake, serving out to win the title.