Italy's Matteo Berrettini broke down in tears after an abdominal injury forced him to retire from his opening ATP Finals match in front of a passionate home crowd in Turin.

After lengthy on-court treatment, the Wimbledon finalist - who trailed 7-6 (9-7) 1-0 - tried to play on.

But the 25-year-old could only hit one more serve before having to retire.

"The atmosphere was one of the best that I felt in my whole life and that's probably why I feel so bad right now," said Berrettini, who remained hopeful the injury would not rule him out of the tournament.

"The thought of not being able to finish the match was killing me. The worst thing that could have happened, happened.

"It is probably the worst day I've had on a tennis court in my life."

Zverev, who clapped his opponent off court and drew a sad face on a television camera lens, said he "felt like crying" at Berrettini's misfortune.

"The tennis is not important, the most important thing is both players can shake hands at the end of the match and are healthy," he added.

If Berrettini is not fit enough to continue in the season-ending event then Jannik Sinner - another Italian player - would replace him as the first alternate.

British number one Cameron Norrie is the second alternate in Turin, which is hosting the event for the first time after taking over from London.

Earlier, Russian second seed Daniil Medvedev - the defending champion - won 6-7 (5-7) 6-3 6-4 against Polish debutant Hubert Hurkacz in the opening match of the event.

At the WTA Finals in Mexico, third seed Karolina Pliskova put herself within touching distance of the semi-finals with a comeback three-set win against fellow Czech Barbora Krejcikova.

Pliskova saved two break points in the penultimate game and then took French Open champion Krejcikova's serve to earn a 0-6 6-4 6-4 victory.