Defending champion Daniil Medvedev recovered from a set down to defeat Poland's Hubert Hurkacz on Sunday on the opening day of the ATP Finals in Turin but said he had felt "zero pressure".

The Russian second seed triumphed 6-7 (5/7), 6-3, 6-4 over seventh seeded Hurkacz.

"I had zero break points to save, so in a way I was never under pressure," said US Open champion Medvedev who fired 15 aces in the match.

"I had only two games on Hubert's serve where I had break points. Super happy to make this work and that I managed to get both breaks."

Hurkacz has enjoyed a breakout season, winning his first Masters event in Miami and making the semi-finals at Wimbledon after beating Roger Federer in the last-eight.

He is appearing in the end-of-season showpiece for the first time.

"I'll just try to stay positive," he said.

"I lost this match, but I'll think about the positives and what I can do better and prepare for the next one."

Medvedev has now won 23 of his last 26 matches.

Later Sunday, sixth-seeded Matteo Berrettini of Italy saw his challenge suffer a painful end when injury forced him to retire against Alexander Zverev.

The German third seed was 7-6 (9/7), 1-0 ahead when Wimbledon runner-up Berrettini called it quits with an apparent abdomen strain.

"I don't really know what to say, because this is the worst feeling a player can have," Zverev, the 2018 ATP Finals champion, said in his on-court interview.

"You play all year long to qualify for this beautiful tournament. For Matteo playing at home, this is the worst feeling I think he will ever have in his career."

Berrettini, who described his retirement as his "worst moment on a tennis court" has not yet pulled out of the event.

Should he decide to do so, then he would be replaced by another Italian in the shape of alternate, Jannik Sinner.

This year's ATP Finals are being played in Turin for the first time after being switched from London.

On Monday, world number one Novak Djokovic faces Casper Ruud while Stefanos Tsitsipas tackles Andrey Rublev.