LONDON — On the verge of an almost flawless performance against Matteo Berrettini at the ATP Finals, Novak Djokovic still found a reason to berate himself.

Leading 4-0 in the second set on Sunday, Djokovic hit a backhand wide to concede a break to the Italian — the only blip in an otherwise dominant 6-2, 6-1 victory — and then angrily turned toward his player's box and shouted out his frustration.

Even at the end of another grueling season, Djokovic won't tolerate any dips in his game as he pursues a record-equaling sixth ATP Finals title.

"There's no other reasonable explanation (to that reaction) than trying to play as perfect as possible," Djokovic said. "But I guess that's me, you know."

It was certainly the same Djokovic that the O2 Arena crowd has become used to seeing. Although the second-ranked Serb — who won four straight titles at the year-end event in London between 2012-15 — only needed to hit a total of 10 winners in the match as Berrettini's 28 unforced errors handed him an easy victory to start round-robin play.

Berrettini's forehand was responsible for 18 of those errors, including two shots into the net to hand Djokovic the only two breaks of the first set.

After giving up that lone break to make it 4-1 in the second, Djokovic broke right back and then clinched the victory with an easy forehand putaway at the net.

"He missed a relatively easy forehand for him that gave me a break (in the first set)," Djokovic said. "And after that I started reading his serve better."

Berrettini was making his debut at the event for world's top eight players and had never faced Djokovic before. He also said he struggled a bit with the speed of the indoor hard court at the O2.

"The ball is not jumping a lot, so it's tough to mix, especially my forehand," Berrettini said. "I think it's better when I have a little bit more time and the ball takes more spin. . I think I started the match pretty good, but he was just better than me today."

Roger Federer, who holds the record of six ATP Finals titles, was set to play Dominic Thiem later in the evening match.

The other four-man group features Rafael Nadal, defending champion Alexander Zverev, Stefanos Tsitispas and Daniil Medvedev. They begin play on Monday.

Djokovic has a chance to overtake Nadal for the year-end No. 1 ranking with a strong showing in London. He trails the Spaniard by 640 ranking points, with a maximum of 1,500 points up for grabs.