Tsitsipas led 5-3 in the tie-break of the first set but then lost four points in a row to put himself at a disadvantage.
London: World number two Rafael Nadal advanced into the second round of the ATP Finals by 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 on Sunday as per Xinhua news agency. "I played solid with my serve. I didn't suffer much, and that helps to play more relaxed on the returns," he added.
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 tie-break 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. It 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 number one 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.
"It's important to start well, of course, for the confidence, and winning straight sets helps," said Nadal on Sunday as per Xinhua news agency. "I played solid with my serve. I didn't suffer much, and that helps to play more relaxed on the returns," he added.
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 tie-break 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. It 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 number one 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.
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