Novak Djokovic. Image Credit: AFP

Novak Djokovic stayed unbeaten in the round-robin stage of the ATP Finals as he overcame Russia's Daniil Medvedev 6-3 6-7(5) 7-6(2) in a gruelling clash at the Pala Alpitour in Turin on Friday to top the Red Group.

Djokovic, who is chasing a sixth title at the season-ending championship to match Roger Federer's record, drew first blood in a tight opening set to take a 5-3 lead and consolidated the break for the early advantage in the match.

Trailing 5-4 in the next, the Serbian saved three set points and drew level with a blistering cross-court shot at the end of a 26-shot rally but appeared to struggle physically in losing the tiebreak as he allowed Medvedev back into the match.

Having already reached the semi-finals after beating Andrey Rublev on Wednesday, some might have expected Djokovic to ease up and save his energy for Saturday's semi-final meeting with Taylor Fritz but the 35-year-old responded in style.

The Serbian battled back from a break down in the decider to hand Medvedev his third straight defeat after three hours and five minutes.

"Daniil and I have had some battles in the past and I knew coming into the match it was going to be his last of the season and he wouldn't want to finish with a loss," Djokovic said.

"I started off well, had chances in the second set, had break points but he played very well in those decisive points. He was playing really quick. I didn't feel physically well in the third, struggled a lot. It's a battle and it's a fight.

"Being able to find the last drop of energy to come back ... at 4-5 (in the third set) I managed to read his serve and anticipate well and got myself in a good position." Greek Stefanos Tsitsipas meets Russian Rublev later on Friday, with the winner facing Norway's Casper Ruud in the semis.

Rublev joins Djokovic in Turin ATP Finals quartet

Andrey Rublev sealed a place in the final four of the Turin ATP Finals on Friday rallying from a set down to beat Stefanos Tsitsipas 3-6, 6-3, 6-2.

Rublev's win saw him overtake Tsitsipas in the Red Group to take second place behind Novak Djokovic.

Novak Djokovic marked himself out as the man to beat when he remained unbeaten earlier Friday producing a battling 6-3, 6-7 (5/7) 7-6 (7/2) win over Daniil Medvedev.

The 21-time Grand Slam winner Djokovic went into the dead rubber already assured of his place in the semis after wins over both Rublev and Tsitsipas.

While Medvedev in contrast was already eliminated after losses in both his outings on the hardcourt in Turin.

With the Group stage now over Djokovic faces the American Taylor Fritz on Saturday with the second direct elimination semi-final to be contested between Norway's Casper Ruud and the 25-year-old Rublev.