London — Stefanos Tsitsipas rallied to beat Dominic Thiem 6-7 (6), 6-2, 7-6 (4) on Sunday to become the youngest ATP Finals champion in 18 years and collect the biggest title of his career.

The 21-year-old Greek bounced back from dropping a tight first set in the final by racing out to a 4-0 lead in the second, and then held off his Austrian opponent's comeback in the third.

Tsitsipas couldn't capitalize on another early break and a 3-1 lead in the deciding set but won the last three points of the tiebreaker, clinching the win when Thiem sent a return wide.

"I have no clue how I played so well in the second set," Tsitsipas said. "It was pretty frustrating for me to be playing with such nerves for the first time in such a big event. I was a break up (in the third set), I couldn't manage to hold it. Things were decided in the tiebreaker and I am so relieved by this outstanding performance and fight that I gave out on the court."

Tsitsipas, who was making his first appearance at the season-ending tournament for the world's top eight players after a breakthrough season, beat six-time champion Roger Federer in Saturday's semifinals.

He is the youngest champion at the ATP Finals since Lleyton Hewitt in 2001.

Thiem beat both six-time champion Roger Federer and five-time winner Novak Djokovic in the group stage but lost another big final after twice finishing runner-up to Rafael Nadal at the French Open.

"It was so close," Thiem said. "But that's how it is in tennis."

It is the fourth year in a row that there is a first-time champion at the ATP Finals, following wins by Andy Murray in 2016, Gregor Dimitrov in 2017 and Alexander Zverev last year.

Earlier, French duo Nicolas Mahut and Pierre-Hugues Herbert won the doubles title by beating Raven Klaasen of South Africa and Michael Venus of New Zealand 6-3, 6-4.