Daniil Medvedev came from behind to beat US Open champion Dominic Thiem in a gripping encounter and win the biggest title of his career at the ATP Finals in London on Sunday.
The Russian fourth seed was second best for large periods at the empty O2 Arena but found a way back into the contest and prevailed 4-6, 7-6 (7/2), 6-4.
Medvedev, who beat five-time champion Novak Djokovic and world No 2 Rafael Nadal earlier in the week, is the first man to beat all three of the world's top-ranked players at the season finale.
The giant Russian, 24, has now won 10 consecutive matches following his title earlier this month at the Paris Masters and his victory will taste sweet a year after a winless debut in London.
But it is a painful defeat for Thiem, who also lost in the final of the elite eight-man event last year, to Stefanos Tsitsipas.
"What a match," said Medvedev. "One of my best victories, two hours and 42 minutes, three sets against an amazing player.
"Dominic, congratulations already for what you achieved in your career. Your name is already in the history of tennis books.
"It is amazing. You won a Grand Slam this year. You are playing unbelievable. I hope we are going to have many more matches to come on the big occasions like this."
Thiem weathered early pressure from Medvedev and secured the first break of the match in the fifth game, when the Russian served a double fault.
He sealed the set in fortunate fashion when his forehand clipped the net and looped over Medvedev's racket.
Medvedev, who came from a set down against Nadal in Saturday's last-four contest, made a solid start to the second set.
But Thiem held to love in the fourth game, looking comfortable as the giant Russian sought a way back into the match.
The 27-year-old third seed had chances to earn a potentially decisive break but they slipped by and he yelled in frustration as he squandered his opportunities.
The Austrian took a tumble onto the blue court at the start of the eighth game and found himself break point down but toughed it out, with Medvedev looking stronger as the set wore on.
The second set went to a tie-break. Having recovered from 0-4 to win the deciding tie-break against Djokovic on Saturday, Thiem got a taste of his own medicine as Medvedev turned a 2-0 deficit into a 7-2 victory.
The Austrian appeared to be struggling to cope with the punishing pace in the third set and Medvedev did not give him the rhythm to get his big groundstrokes flowing.
He finally cracked in the fifth game, when the Russian produced a volley at the net to break.
Thiem remained under pressure and had to battle hard to avoid going down another break.
But all Medvedev now had to do was keep his nerve. He did so as he served out for victory.
"Of course, I am disappointed, but at the same time I am also proud of the performance of all the week," said Thiem. "Daniil really deserved it. It was an amazing match."
Medvedev is the first Russian to win the ATP Finals since 2009, when Nikolay Davydenko triumphed at the first edition of the season finale held in London. Next year, the tournament moves to Turin in Italy.