LONDON, Nov. 10 (Xinhua) -- The six-time champion Roger Federer lost to Dominic Thiem 7-5, 7-5 in his first match at the ATP World Tour Finals here on Sunday after world No. 2 Novak Djokovic downed Italian debutant Matteo Berrettini 6-2, 6-1.

Federer, 38, was broken in the opening game but managed to take revenge on his 12-year younger opponent in the fourth game. At 5-5, the Swiss legend struggled a little bit as Thiem kept putting pressure on his backhand. The Austrian won the 11th and 12th games to make it 7-5 in the first set.

The second set saw the duo played equally matched until Thiem broke in the 11th game to wrap it up in 7-5 again.

"The start definitely didn't help. That put me on the back foot a little bit," said Federer, who lost to Thiem five times in seven meetings between them. "But I recovered well, and I thought after that the match was actually pretty even for a long period of time."

Earlier, the five-time champion Djokovic gave Berrettini little chance on the court, grabbing the victory after 64 minutes in the Group Bjorn Borg opener.

The 23-year-old Berrettini thought he played well on Sunday, but his rival was just much better.

"We were playing flat, fast, and he was moving better than me, was playing better than me. So it was tough to win the points," said Berrettini.

Federer is set to face off against Berrettini in his second group match on Tuesday while Djokovic is slated to meet Thiem the same day.

"It's a normal tournament from here on forward, not allowed to lose anymore for me," said Federer. "I've got to recover, I've got make sure I play better than today, and hopefully I can win that match."

The first round matches of Group Andre Agassi will be unveiled Monday with Russian Daniil Medvedev facing Greek Stefanos Tsitsipas before world No. 1 Rafael Nadal plays against defending champion Alexander Zverev. Enditem