Djokovic, Nadal vie for year-end number one spot at ATP Finals

London: Novak Djokovic can end his season with a flourish by equalling Roger Federer's record haul of six titles at the ATP Finals and pipping Rafael Nadal to the year-end number one spot.

Germany's Alexander Zverev shocked Djokovic in last year's final at London's O2 Arena but the Serbian will start as the firm favourite after cantering to victory at last week's Paris Masters.

Djokovic and Nadal have shared the spoils at this year's four Grand Slams, refusing to yield to the next generation.

Nadal, who has never won the ATP Finals, dethroned his old rival from the top of the rankings this week and will be hoping to seal the year-end spot despite injury doubts ahead of the tournament, which starts on Sunday.

Federer, Nadal and Djokovic were the top three seeds when they first competed together at the tournament way back in 2007. Twelve years later little has changed at the top of men's tennis, although they will be wary of the challenge from Russia's Daniil Medvedev, who has won two Masters titles since August and pushed Nadal to five sets in the US Open final.

Austria's Dominic Thiem, Stefanos Tsitsipas of Greece, Germany's Zverev and Italian Matteo Berrettini make up the rest of the field of eight players.

Nadal is bidding to clinch the year-end number one spot in the ATP rankings for a fifth time -- he is currently 640 points ahead of Djokovic -- but comes into the tournament under a cloud.

The Spaniard said this week that he had had a scan in Mallorca after straining an abdominal muscle and would work on adapting his serve but he remained bullish about featuring at the ATP Finals after missing out last year with an injury.

"The idea is to be able to play the #atpfinals in London," tweeted Nadal, who has often been blighted by late-season injuries.

Nadal's opponents in the Andre Agassi group are Medvedev, Tsitsipas and Zverev while Djokovic will be up against Federer, Thiem and Berrettini in the Bjorn Borg group.

Djokovic won four titles in a row from 2012 to 2015 but has been beaten in the final in two of the past three years.

Speaking after his Paris Masters win about his chances of claiming a record-equalling sixth year-end number one finish, Djokovic said: "There's always a chance that I win all my matches in London, that I play well.