Thiem outlasted Tsitsipas 7-6 (5), 4-6, 6-3.
In last year's ATP final, Tsitsipas beat Thiem in a third-set tiebreaker to clinch the biggest title of his career.
“He dealt with my serve better and took time [away], he stepped back and applied the pressure,” Tsitsipas said after the match.“He put more pressure on me. Some of the second serves that I had early in the third set… and I think that paid off for him.”
Tsitsipas may still be able to advance depending on his results against Nadal and Rublev in the last two group-stage rounds.
ATP Finals 2020
Due to the pandemic, there are no fans inside the 20,000-capacity venue. There are also no line judges, as the tournament is using electronic line calling for the first time. The system means players can no longer challenge whether a ball was in or out as those calls get made automatically. However, they can request a video review for other disputed situations, such as double bounces.