Dominic Thiem saw off Stefanos Tsitsipas in three sets at the ATP Finals to avenge his defeat in last year's final.
The Austrian third seed, hoping to cap a memorable 2020 that produced his first Grand Slam at the US Open, came through 7-6(5) 4-6 6-3.
Thiem was pegged back in the second set by Tsitsipas but raced into a 3-0 lead in the decider.
And after winning a mammoth deuce to earn a 4-1 lead, the 27-year-old stayed resolute on serve to get his bid in London off to the ideal start.
"You cannot have any slow starts here," Thiem said.
"I have experienced in the past four years how important it is to have a fast start and I'm happy I did it."
Rafael Nadal plays Russian debutant Andrey Rublev in the other opener in Group London 2020 later on Sunday.