LONDON, Nov. 11 -- 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.

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