Tsitsipas rallied to beat 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.
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 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.
Prime Minister Kyriakos Mitsotakis, who is in London, attended the match and congratulated Tsitsipas afterwards.