LONDON - World number two Rafael Nadal won his opening match at the ATP Finals here on Sunday, dispatching Russia's Andrey Rublev 6-3, 6-4 in one hour and 18 minutes.

"It's important to start well, of course, for the confidence, and winning straight sets helps," said Nadal, who triumphed in the French Open for a record 13th time this year. "I played solid with my serve. I didn't suffer much, and that helps to play more relaxed on the returns."

The first match of Group London 2020 was played between last year's runner-up Dominic Thiem and winner Stefanos Tsitsipas. Austrian third seed Thiem avenged that defeat by beating the Greek 7-6 (7-5), 4-6 and 6-3.

Tsitsipas led 5-3 in the tiebreak of the first set but then lost four points in a row to put himself at a disadvantage.

"That was the silliest thing that ever happened to me," Tsitsipas said of his play while leading 5-3. "I had a ball over the net, and I decided to play forehand instead of a smash. I don't know what kind of decision that was, but it was not sure enough. Didn't take time. Just rushed."

Nadal will play against newly-crowned US Open champion Thiem in his second group match on Tuesday.

"I'm going to try to be ready for it," said Nadal "It's going to be a big challenge, but I really hope the victory of today could help me for that match."

Novak Djokovic, who received the year-end No. 1 trophy for a record-equaling sixth time, will start his campaign in Group Tokyo 1970 with a clash against Diego Schwartzman of Argentina on Monday.