LONDON -- Three years after coming to the ATP Finals as a hitting partner to the top pros, Stefanos Tsitsipas is leaving the tournament as champion.
Tsitsipas rallied to beat Dominic Thiem 6-7 (6), 6-2, 7-6 (4) on Sunday for the biggest title of his career, becoming the youngest champion at the season-ending event in 18 years.
"I remember myself watching this event on TV and thinking, 'Oh, these guys have done an insane year to be playing here,'" said the 21-year-old Tsitsipas, who was making his first appearance at the tournament. "And now I'm in the position to be champion, so it feels awesome."
It's been quite a journey for Tsitsipas, who first met Thiem at the O2 Arena in 2016 when he was the world's top-ranked junior and was invited to be a hitting partner for the ATP Finals participants.
"That is unbelievable," Tsitsipas said when reminded of that day. "I just remembered [that], first time I met Dominic was I came here as a sparring partner. I think my first hit was with Dominic. It's unbelievable, isn't it? We are now facing each other in the final."
It was quite a final, too.
The 21-year-old Greek bounced back from dropping a tight first set by racing out to a 4-0 lead in the second, and then held off the Austrian'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 missed some very close balls in the tiebreak," Thiem said. "But I cannot do anything now about it. It was a great match from both of us."
The win caps a breakthrough season for Tsitsipas, who reached the semifinals of the Australian Open, won two ATP titles and became the first Greek man to break into the top 10.
He also leaves London with $2.6 million in prize money for this win and joins a group of young players -- including last year's ATP Finals champion Alexander Zverev and US Open runner-up Daniil Medvedev -- who are seen as the top up-and-coming talents and challengers to the sport's Big Three of Roger Federer, Novak Djokovic and Rafael Nadal.
The Greek has already grown quite the fan base, partly because of a Bjorn Borg-style haircut and partly because of an aggressive playing style that is a bit old-school with frequent forays to the net.
"He's great for tennis, because he has a very attractive game style, one-handed backhand, comes in a lot," Thiem said. "It's great that he's up on the top. It's great that he's going to fight for the big titles in the future. I'm 100% sure of that. I'm also very sure of the fact that I can challenge him in every single match we're going to play. I really hope that we're going to have a lot of big matches coming, the two of us."
Thiem beat both Federer and five-time winner Djokovic in the group stage but lost another big final after twice finishing runner-up to Nadal at the French Open.
This one was much closer than his losses to Nadal, though, which made it tougher to swallow.
"That's why it's probably mentally the most brutal sport existing, because you can play such a great match and end up losing in the championship match," Thiem said. "From that point of view, it's a very disappointing loss, very hard to digest. But on the same hand, I had some amazing wins also."
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, Grigor Dimitrov in 2017 and 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.