Serbia's Novak Djokovic celebrates after defeating Norways' Casper Ruud during their ATP World Tour Finals singles tennis match, at the Pala Alpitour in Turin on Nov 15, 2021. (LUCA BRUNO / AP)

World number one Novak Djokovic began his quest for a sixth title at the season-ending ATP Finals with a 7-6(4) 6-2 victory over Norway's Casper Ruud in their round robin encounter in Turin 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.

Casper Ruud's powerful forehand helped him keep pace with Novak Djokovic for much of the first set, but he was outplayed in the tiebreak as the 20-times major winner forced Ruud into a string of errors to take the opener

"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 tiebreak 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 91 minutes.

Djokovic was later presented with the year-end world number one trophy for a record seventh time, bettering the mark set by 14-times Grand Slam champion Pete Sampras.

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Norways' Casper Ruud returns the ball to Serbia's Novak Djokovic during their ATP World Tour Finals singles tennis match, at the Pala Alpitour in Turin on Nov 15, 2021. (LUCA BRUNO / AP)

"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.

In the day's second singles match, Greek world number four Stefanos Tsitsipas suffered a disappointing start as he was beaten 6-4 6-4 by Russia's Andrey Rublev.

Rublev, who made his debut in the tournament last year when he lost a group match to Tsitsipas, has struggled of late for form but looked back to his best on Monday.

He did not even offer one break point opportunity as he claimed victory in 90 minutes.

"When you serve good, it's always the key," Rublev said. "But it was not even just the serve. I was returning really aggressive, from the baseline I was also really aggressive. Today I played a great match and I'm really happy."

Tsitsipas, the champion in 2019, had been troubled by an arm injury and refused to say afterwards whether it was still causing him problems against Rublev.

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The French Open runner-up will need to bounce back against Ruud on Wednesday to stay in the hunt for a semi-final place.