LONDON - World No 1 Rafael Nadal failed to resist an on-fire Alexander Zverev here on Monday, losing his first group match in straight sets 6-2, 6-4 to the German defending champion at the ATP World Tour Finals.

Nadal, leading their head to head by 5-0, could hardly match Zverev, who broke the Spaniard three times with aggressive returns and dominated his own games with powerful serves.

"Today was definitely a great match," said the 22-year-old Zverev. "I'm super happy to have this match the first day that I played."

Nadal pulled out of Paris Masters earlier this month due to abdominal injury and just resumed serving in practice last Thursday after he arrived in London. But the 33-year-old refused to pin his loss on the injury.

"It was not a problem with the abdominal at all. I did not feel pain in the abdominal at all," said Nadal. "I am disappointed about my competitive spirit, because I was not able to bring the game to that moment that I needed."

In the opener of Group Andre Agassi earlier, Stefanos Tsitsipas from Greece beat world No. 4 Daniil Medvedev from Russia 7-6(5), 6-4.

Both are debutant in the year-ending tournament and Medvedev led the head-to-head 5-0 before the match. But the 23-year-old Russian believed that he was "mentally missing something" in their sixth meeting.

"I think just general energy was not the way I wanted. And talking about energy, I'm not talking only about physical. Mentally I was missing something. I didn't have good energy enough to get the win today," said Medvedev.

Tsitsipas, 21, said defeating Medvedev to win his first match in the Finals meant more than extra. "It's a victory that I craved for a long time and it's great that I came in at this moment."

On Tuesday, Swiss legend Roger Federer will play against Italian rookie Matteo Berrettini who lost his opening match to Novak Djokovic. Federer dropped his first match in Group Bjorn Borg to Dominic Thiem.