Novak Djokovic ignored an annoying wave of sickness in the final set to break the will of Daniil Medvedev 6-3, 6-7(5), 7-6(2) on Friday to complete a 3-0 sweep of group matches at the ATP Finals.
He will play a semi-final on Saturday against American Taylor Fritz, a year-end tournament debutant.
Five-time champion Djokovic scrambled to stay in contention as Medvedev – who ended up losing all three of his matches 7-6 in the third set – served for a victory.
Djokovic broke for 5-all with a volley winner and a game later took the match into a deciding tiebreaker.
He wasted no time in finishing off the win despite feeling poorly, running out a 5-1 lead in the decider and eventually firing a winner to end it after nearly three and a quarter hours.
“Today was one of the toughest matches I’ve played this year, I didn;t feel well in the third set,” the winner said.
“It was a struggle, but that happens in sport. Daniil served for the win but I managed to find the right shots.
“We both fought to the end.”
Djokovic added: “I’m very content with this win, it was more than just a match to me.”
The Serb overcame 16 aces from Medvedev while breaking twice.
“This is a huge relief,” he said. “I knew I would have a battle as this was his last match of the season and he would not want to finish with a loss.
“I’m proud I was able to find my last drop of energy and focus to come back into the match at 5-5
“I never thought of withdrawing, it was really important for me to win.”