Reigning champion Daniil Medvedev moved to the brink of the last four at the ATP Finals on Tuesday by edging out Alexander Zverev 6-3, 6-7 (3-7), 7-6 (8-6) in a dramatic match in Turin.

Russian second seed Medvedev missed two match points in a gripping deciding-set tie-break before getting over the line at the third time of asking.

"Definitely one of the matches to remember," said the US Open winner, who beat Poland's Hubert Hurkacz in his opening group encounter.

Medvedev now leads Zverev 6-5 in their head-to-head, having also beaten him in the round-robin stage last year.

"Sometimes that's how tennis is," he added. "You just try to do your best. Sometimes luck is on your side, sometimes not."

Medvedev will be guaranteed a semi-final place if Hurkacz beats Jannik Sinner later on Tuesday.

Victory for Hurkacz would set up a winner-takes-all final Red Group clash with Zverev.

Italian rising star Sinner replaced fellow home hope Matteo Berrettini in the draw. The Wimbledon runner-up withdrew due to the abdominal injury which forced him to retire from his first match against Zverev early in the second set.

In front of a large crowd at the Pala Alpitour arena on Tuesday, Medvedev edged the first set thanks to a break in Zverev's opening service game and also saved three break points himself.

World number three Zverev appeared angered by a line call late in that set, before channelling his energy to take the second in a tie-break and level the match.

Medvedev saved a break point in the 11th game of the deciding set to help force another breaker, in which he battled back from 4-2 down to lead 6-4.

He saw two match points come and go but finally clinched a vital win after more than two-and-a-half hours on court.

"That was a match to remember, just amazing," conceded Zverev.

Destroyed' Berrettini pulls out 

The 20-year-old Sinner will make his ATP Finals debut against Hurkacz after Berrettini pulled out of the season-ending event, being staged in Italy for the first time after moving from London.

"I thought, cried, and finally decided," Berrettini said in a post on social media.

"My Finals end here. I am destroyed. I never thought I would have to give up the most important tennis event ever held in Italy in this way.

"The truth is that as much as I wanted to play in front of you once again, I felt and therefore decided that my body is not ready to face the challenges that lie ahead."

Novak Djokovic leads the Green Group after beginning his attempt to equal Roger Federer's record number of ATP Finals triumphs with a straight-sets win over Casper Ruud on Monday.

Russian Andrey Rublev also impressed with a 6-4, 6-4 victory over 2019 champion Stefanos Tsitsipas and sits behind Djokovic in the standings by virtue of winning one game fewer than the 20-time Grand Slam champion.