Daniil Medvedev came from behind to beat Dominic Thiem and win the Association of Tennis Professionals (ATP) Finals in London - the biggest win of his career to date.
The Russian entered the final as the only player not to have lost a match at the event and left it with that record intact.
Medvedev triumphed 4-6, 7-6 (7-2), 6-4 to win the ATP Finals in only his second appearance at the event.
It was a tight match and Medvedev won only three more points than Thiem, although the victory could have been more comfortable were it not for Medvedev's profligacy with break points.
The 24-year-old converted just one of nine.
Victory over Thiem means that Medvedev beat all of the world's top three this week, having defeated Spain's 20-time Grand Slam champion Rafael Nadal in the semi-finals and overcome world number one Novak Djokovic of Serbia - a five-time winner of this event - in the group stage.
Medvedev is also the last player to win the end-of-season finale at London's O2 Arena, as the tournament is due to move to Turin next year.
"I always said before this tournament that it would be an amazing story if, here in London, where the tournament was for 11 years, that the first champion would be Russian and the last champion would be Russian, too," Medvedev said.
"A lot of thanks to [2009 champion] Nikolay Davydenko for being an inspiration for many kids [like] me [by] winning here.
"I hope to continue doing his job."
Austrian Thiem beat Medvedev earlier this year in the US Open semi-finals, en route to winning a first Grand Slam title.
Thiem, the world number three, has been the runner-up at this event in each of the last two seasons.
Croatian Nikola Mektić and Dutchman Wesley Koolhof triumphed in the doubles final.
They beat Austrian-French duo Jürgen Melzer and Édouard Roger-Vasselin 6-2, 3-6, 10-5.