LONDON — Dominic Thiem made sure this year's ATP Finals didn't begin the same way last year's ended. 

Thiem outlasted Stefanos Tsitsipas in a rematch of the 2019 final, beating the defending champion 7-6 (5), 4-6, 6-3 Sunday in the opening singles match of the tournament inside a largely empty O2 Arena. 

The U.S. Open champion broke Tsitsipas for a 2-0 lead in the third set with a drop shot at the net and converted his second match point when Tsitsipas sent a forehand wide. 

Tsitsipas beat Thiem in a third-set tiebreaker in last year's final to clinch the biggest title of his career at the season-ending event for the world's top eight players. But he let the first-set tiebreaker slip away this time, after leading 4-1 and 5-3. Facing set point, the Greek failed to put away a fairly simple overhead at the net before sending a lob long to hand Thiem the lead. 

Tsitsipas broke for a 2-1 lead in the second set with a forehand passing shot before leveling the match. He had a chance to break back at 3-1 in the third but netted a backhand, letting out a scream of frustration after the miss. Thiem eventually held serve after a game that went to six deuces, and was never troubled the rest of the way. 

"That was crazy, I was pretty tight I must say in that game," Thiem said about his hold for 4-1. "I think that was the last key moment where the match went finally in my direction." 

This is the 12th and last year that the ATP Finals is played at the 02 Arena before moving to Turin, Italy, next year. Because of the coronavirus pandemic there are no fans inside the 20,000-capacity venue this time — meaning the normally raucous atmosphere has been replaced by an eerie silence interrupted only by muted applause from the players' boxes between points. 

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. 

Rafael Nadal was set to take on newcomer Andrey Rublev in the late match. Tsitsipas can still advance depending on his results against Nadal and Rublev in the last two group-stage rounds. 

Before the opening match, Novak Djokovic was presented with a trophy on court for finishing the year with the No. 1 ranking for a record-equaling sixth time. 

On Monday, Djokovic will face debutant Diego Schwartzman before Daniil Medvedev takes on 2018 champion Alexander Zverev.