LONDON - World number one Novak Djokovic overcame a slow start to defeat Argentinian debutant Diego Schwartzman 6-3, 6-2 at the ATP Finals on Monday.

In the opening match in Group Tokyo 1970, the 33-year-old Serbian was broken first in the third game but managed to break back twice later to win the first set 6-3.

Djokovic showed his dominance on court in the second set, clinching his 12th straight victory in opening matches in the season finale.

Despite the absense of spectators, Djokovic still made the gesture of showing his heart to all four sides of the court after his win as if there were people cheering for him.

"That's my celebration. That's my also gratitude to the court and to this opportunity to be able to compete," said Djokovic. "Even though there was no crowd in the stands, I know there were a lot of people watching it on TV, so that was me sharing that emotion with them."

The five-time Finals champion will meet in-form Daniil Medvedev next, after the Russian fourth seed beat Alexander Zverev of Germany 6-3, 6-4 in the other group match on Monday.

Eight days after defeating Zverev to clinch the Paris Masters title, Medvedev outplayed the former Finals champion again in London to claim his first victory at the ATP Finals.

"Confidence is the key, for sure. Winning a Masters always helps for the confidence," said Medvedev who lost three matches at last year's Finals.

World number two Rafael Nadal will play his second match in Group London 2020 against Austrian Dominic Thiem on Tuesday.

In the first round matches on Sunday, the Spaniard breezed past Russian debutant Andrey Rublev 6-3, 6-4, while Thiem avenged last year's final defeat by beating defending champion Stefanos Tsitsipas 7-6 (7-5), 4-6, 6-3.