Novak Djokovic produced a masterclass on Monday to launch his campaign for a record-equalling sixth title at the ATP Finals as Daniil Medvedev outfought former champion Alexander Zverev.
The world number one crushed Argentine eighth seed Diego Schwartzman 6-3, 6-2 in the opening round-robin match in Group Tokyo 1970 before Medvedev triumphed 6-3, 6-4.
Djokovic, who has already sealed the year-end number one spot for a record-equalling sixth time, is aiming to match the absent Roger Federer, who holds the record of six ATP Finals wins.
The Serbian, 33, has enjoyed another sparkling season despite the coronavirus interruption, winning four titles including the Australian Open.
He came into the match at an empty O2 Arena with a 5-0 record against the diminutive Schwartzman but the Argentine debutant started impressively, breaking Djokovic in the third game.
However, the top seed broke back immediately and upped his game, breaking again in the eighth game and converting his first set point with a powerful serve down the T.
Two more breaks in the second set were enough to take Djokovic to a comfortable win.
The top seed admitted he had started slowly but said he was pleased with his game as the match wore on.
"In the second set I started swinging through the ball a bit more, bit less hesitation from both corners and I was very pleased with the way I played in the second set especially," he said.
The five-time champion said he was missing the fans, who usually pack the O2 Arena.
"I've been playing the ATP Finals in this arena for many years and I've been blessed to experience some incredible matches and atmospheres and it feels very strange to play in front of empty stands," he said.
The evening contest between Medvedev and Zverev was a rematch of last week's Paris Masters final, when the Russian came out on top.
Both players struggled to hold serve in a intense start to the contest, swapping breaks at the start of the match as they appeared to struggle for breath.
Medvedev, who lost all three matches on his first appearance at the ATP Finals last year, broke again in the sixth game and that proved enough for him to take the first set.
The pair settled into more of a rhythm on serve in the second set but 2018 champion Zverev blinked first, the German broken in the seventh game.
Russian fourth seed Medvedev said his win at the Paris Masters had given him confidence, describing Monday's battle as exhausting.
"The intensity of the match was one of the most I had in my career so actually second set for both of us was easier to make some winners because we were both tired so we couldn't run any more.
"There were some amazing rallies. I'm happy that I came out on top of this battle."
World number two Rafael Nadal, chasing his first ATP Finals crown, won his first match in Group London 2020 on Sunday, along with US Open Dominic Thiem.
The O2 Arena is hosting the ATP Finals for the 12th and final season before the year-end championships moves to Turin next year.