World No.1 Novak Djokovic began his quest for a sixth title at the season-ending ATP Finals with a 7-6 (7-4), 6-2 victory over Norway’s Casper Ruud in their round robin encounter here on Monday.

Djokovic suffered a fall and lost his opening service game but recovered to find his rhythm against Ruud, who is making his debut at the event following five tournament wins this season.

“It was strange. I played a good forehand and came into the net and had a comfortable overhead. I slipped, tripped, dropped my racquet and lost my serve,” Djokovic said.

“The conditions here are tough. If you lose your serve, it’s difficult to get it back. But I managed to stay in. I’m pleased with the way I handled the match.”

Ruud’s powerful forehand helped him keep pace with Djokovic for much of the first set, but he was outplayed in the tie-break as the 20-times major winner forced his 22-year-old opponent into a string of errors to take the opener.

It was plain sailing in the second set for Djokovic, who broke in the first game and dominated the longer rallies as Ruud struggled on serve before eventually going down in an hour and 31 minutes.

Djokovic was later presented with the year-end world No.1 trophy for a record seventh time, bettering the mark set by 14-times Grand Slam champion Pete Sampras.

“I was four or five when I saw Sampras play at Wimbledon for the first time. I asked for a racquet and fell in love with tennis. To know that I hold this record now, after Pete, it's incredible,” said the 34-year-old.

Berrettini injured

Italy’s Matteo Berrettini broke down in tears after an abdominal injury forced him to retire from his opening match against Alexander Zverev, casting doubt over his participation for the rest of the season-ending tournament.

Olympic champion Zverev was awarded a win after Berrettini pulled out injured early in the second set, with the German leading 7-6 (9-7), 1-0 on Sunday.