World No 2 Daniil Medvedev came from a set down to power past ATP Finals debutant Hubert Hurkacz 6-7(5) 6-3 6-4 and get his title defence off to a winning start in Turin on Sunday.

Olympic champion Alexander Zverev was also awarded a win in his opening match after Italian Matteo Berretini pulled out injured early in the second set with the German leading 7-6 1-0.

Russian Medvedev, who won the US Open title in September for his maiden Grand Slam crown, allowed his Polish opponent to take the early advantage in the season-ending event following a tight tiebreaker but responded in style.

The 25-year-old fired 15 aces to Hurkacz's 12 and converted two break points on a fast court to seal victory in the Red Group ahead of potentially tricky challenges against Germany's Alexander Zverev and Wimbledon runner-up Matteo Berrettini.

"It was a very tight match," said Medvedev, looking to become the first back-to-back ATP Finals winner since Serb Novak Djokovic claimed four between 2012-15, in his post-match interview. "One good shot and the point is over.

"I think a lot of matches are going to be like this because the courts are super fast. The only two games I had break points I managed to break and that was the key."

Medvedev showed glimpses of the form that helped him beat Djokovic, Rafael Nadal and Dominic Thiem en route to the title on London last year, as he went up 3-0 in the second set after an early break.

He raised his level in the third set to close out the match, hitting more than 30 winners and not facing a single breakpoint in a clash lasting a little more than two hours.

World No 3 Zverev was made to work very hard against Berrettini for more than an hour before the Italian pulled out injured.

Berrettini had struggled to hold serve, having to save two break points on his very first service game alone, but his forehand repeatedly got him out of trouble.

Another two dizzying forehand crosscourt winners earned him his first two break and set points at 6-5 but he could not convert either.

Zverev also squandered two set points before clinching the tie break on his third chance after an hour and 19 minutes.

But after a game into the second set Berrettini needed a medical break for on-court treatment to his abdominal muscles and lasted only one more point before pulling out in tears.

"I don't know what to say because it is the worst feeling you will ever have as a player," Zverev said. "Obviously, I feel like crying.

"I thought the first set was incredible high level tennis." (Reuters)