Novak Djokovic booked his place in the last four of the ATP Finals with a 6-4, 6-1 defeat of Andrey Rublev and Stefanos Tsitsipas ended Daniil Medvedev's hopes of winning the season-ending championships in Turin on Wednesday.
After the first nine games went with serve, Djokovic switched on the afterburners to outclass Rublev.
Rublev, who like Djokovic had won his opening match at the year-ender, capitulated when serving to stay in the opener, losing the set as Djokovic thumped a backhand return winner.
The second set was a non-event as five-time champion Djokovic roared to victory in little more than an hour.
"I played one of my best matches of the year," Djokovic, who was watched for the first time by both his children Stefan and Tara, said on court.
"I found the right attitude and the right game and once I won the first set his energy went down.
"In the second set I just wanted to use that momentum and it was a flawless second set."
Despite being guaranteed a place in the weekend's semi-finals when Djokovic will hope to equal Roger Federer's title record at the ATP's season-ender, the 35-year-old Serb said he will not be relaxing against Medvedev on Friday.
"It won't just be a stroll in the park," he said.
"Knowing I'm in the semis it's a great boost but at the same time I want to focus on every match."
If 21-time Grand Slam champion Djokovic wins the title this week he will be the oldest player ever to do so.
Greece's Tsitsipas battled back from 3-5 down in the third set to eliminate Medvedev with a 6-3, 6-7(11), 7-6(1) win to set up a showdown on Friday with Rublev for the Red Group's other semi-final spot.
After capturing the first set behind some outstanding serving, Tsitsipas saw three match points slip away in the second and came one game away from defeat in the decider.
But Tsitsipas would not stray from his fearless brand of tennis, crushing balls from the baseline and charging to the net to seal the rollercoaster win after two hours and 21 minutes at Pala Alpitour Stadium.
"It was very tiring out there," Tsitsipas said.
"I'm so glad I overcame this and I was able to enjoy it with the crowd. It's a great win and I'm extremely proud of the way I fought."
Medvedev pounded 16 aces but his serve deserted him late in the third set and he went down quietly in the final tiebreak, the match ending on a forehand error out wide from the former world number one.
"It was great to get back into the match, I felt reborn, and what a great way to end it," Tsitsipas said.
With the win, Tsitsipas notched his second consecutive win over Medvedev and fourth in 11 total matches.
