Novak Djokovic made a winning start at the ATP Finals as he chases a record-equalling sixth title that will see him return to world number one.

The Serb was imperious, beating Italian eighth seed Matteo Berrettini 6-2 6-1.

World number two Djokovic can overtake Rafael Nadal to become year-end number one this week in London but must reach the final to stand a chance.

Roger Federer plays Dominic Thiem in the same group later on Sunday at the season-ending championships.

The event at the O2 Arena features the top eight players of the year who are split into two groups.

The group stage is a round-robin format with the top two players progressing to the semi-finals with the final held next Sunday.

Nadal is in Group Andre Agassi but is an injury doubt for the tournament with a stomach muscle issue, although the Spaniard said on Friday he is confident of playing against Alexander Zverev in his opener on Monday.

Djokovic makes perfect start

Djokovic is arguably the favourite for the title in London, a title which would equal Federer's wins at the Tour Finals.

He won the Paris Masters last week and continued his fine form against 23-year-old Berrettini, winning in just 63 minutes.

The Italian, making his debut at the event, played well but could not keep up with the relentless Djokovic.

Berrettini regularly served above 130mph, and landed 71% of his first serves, but Djokovic broke serve twice in the first set and three times in the second.

The 2019 Australian and Wimbledon champion was rarely troubled on his own serve and hit just eight unforced errors.

"It was not easy for him playing his first World Tour Finals match," Djokovic said.

"I knew he would be a bit more nervous at the beginning. He has got big weapons with the serve and forehand but I managed to play really solid throughout the whole match."

Briton Salisbury beaten in opener

Earlier, Britain's Joe Salisbury lost the opening match of the doubles competition with his American partner Rajeev Ram.

The fourth seeds lost 6-3 6-4 to Raven Klaasen and Michael Venus.

Londoner Salisbury, who was a 'hitter' at the tournament four years ago, is the only British player in the event.

He and Ram can still progress from the group which also includes second seeds Lukasz Kubot and Marcelo Melo and eighth seeds Ivan Dodig and Filip Polasek.