Austrian third seed Dominic Thiem avenged his defeat by Greece's Stefanos Tsitsipas in last year's final with a three-set win as the ATP Finals began in London for the final time.

Thiem upped the tempo in the decider, hitting 11 winners to see off sixth seed Tsitsipas at an empty O2 Arena.

In the same group, Spain's Rafael Nadal plays Russian debutant Andrey Rublev.

The pair meet in Sunday's evening session looking to match Thiem's opening win in Group London 2020.

World number one Novak Djokovic starts his bid for a record-equalling sixth title on Monday afternoon in Group Tokyo 1970.

Same players, different occasion as London begins an eerie farewell

When Tsitsipas and Thiem met in last year's final, Tsitsipas celebrated winning the biggest title of his career with a large contingent of boisterous Greek fans inside an atmospheric O2 Arena.

Twelve months on, the re-run of that memorable match could not have been played in more contrasting circumstances.

No fans are allowed inside the near 18,000-seater arena because of coronavirus restrictions, meaning London cannot bid farewell to the tournament as it would have wished before it moves to Turin, Italy, next year.

Without the spectators, there was an eerie atmosphere as only the players, officials, ball kids and journalists were allowed inside the dark arena.

Asked what was the main difference for him between the two matches, Tsitsipas sighed and said: "People in the stands."

Initially, the lack of atmosphere did not dilute the level of each player's intensity in a tight opening set.

Both men found their rhythm from the start in strong service games, leading to a tie-break which Thiem nicked after fighting back from 5-3 down with four successive points.

Tsitsipas had not earned a break point in the opener, but decisively took his chance in the third game of the second set and went on to level the match.

Like their meeting 12 months ago, it teed up another deciding set but this time it was Thiem who took control.

The Austrian ramped up his level to move 3-0 in front and, after fighting off a break point and coming through six deuces to hold in the fifth game, sealed what could be a crucial win to his hopes of reaching the semi-finals.

"You cannot have any slow starts here," Thiem said.

"I have experienced in the past four years how important it is to have a fast start and I'm happy I did it."

Djokovic celebrates matching Sampras' number one record

Before the opening singles match, Novak Djokovic was presented with the trophy for finishing as the year-end world number one on the ATP Tour.

The 33-year-old Serb has finished the year as the top-ranked player for a sixth time, equalling the record set by American great Pete Sampras.

"It is very fulfilling and I am proud. I am overwhelmed with happy emotions," said Djokovic on matching the feat of his childhood idol Sampras.

Djokovic's biggest victory of the year came when he won his 17th Grand Slam at the Australian Open in January and he goes into the ATP Finals having lost just three of his 42 matches in 2020.

Salisbury leads British hopes

The finals started on Sunday when debutants Wesley Koolhof and Nikola Mektic fought back to beat German third seeds Kevin Krawietz and Andreas Mies in the doubles.

Fifth seeds Koolhof and Mektic won the final five points in the match tie-break to clinch a 6-7 (3-7) 7-6 (7-4) 10-7 victory.

Britain's Joe Salisbury - the nation's only representative in London - plays later on Sunday.

Salisbury and American partner Rajeev Ram, who are seeded second, face Poland's Lukasz Kubot and Brazil's Marcelo Melo.