Nitto ATP Finals: Rafael Nadal's fitness puts cloud over dream match vs Roger Federer

London: Rafael Nadal and Roger Federer can set up a dream climax to the 2017 season at the ATP World Tour Finals, but organisers are sweating on the Spaniard's fitness and a new crop are itching to spoil the party.

The two all-time greats get star billing in London after each won two Grand Slams and locked down the top two rankings spots as Novak Djokovic and Andy Murray had injury-plagued seasons to forget.

The eight-man field at the O2 Arena has an unbalanced look about it but 20-year-old Alexander Zverev leads a hungry chasing pack, desperate to stake a claim to membership of the game's elite.

World number one Nadal, who won the French Open and US Open this year, admitted on Friday that he is not training at 100 percent ahead of his tournament opener against Belgium's David Goffin on Sunday, although he said he is likely to be fit enough to play.

"I hope. And if I didn't believe I can be ready, for me, I wouldn't be here," said the Spaniard, who will also do battle with Dominic Thiem and Grigor Dimitrov in the 'Pete Sampras' group."I'm working every day, practising well and just trying to be ready for the action."

The 31-year-old, forced to pull out of the Paris Masters last week with a knee injury, is painfully aware that the ATP Finals is the one major piece of silverware missing from his trophy cabinet and he wants to put that right.

"Being here is an important thing but for me the most important thing is everything that happened to me this year, the tournaments, competing almost every week with a very competitive level of tennis, being healthy until Paris," he said.

And Nadal said although he welcomed the opportunity to finish the year with another high-profile clash against Federer, he was not fixated on the need to defeat the Swiss after four straight losses to his old rival in 2017.

Federer, chasing his seventh Tour Finals title, is happy to be back at the end-of-season showpiece after missing out last year during a long layoff. "Last year I couldn't be here so it's nice to be here again because this year I had to start further back in the rankings," said the Swiss, who astonished the tennis world by winning the Australian Open after months away from the sport.

Federer, at the grand old age of 36, said he felt fresh after a season that had exceeded his expectations. "This year I ended up playing much less than I thought I would. Because of the great start to the season I didn't have to push it that much to be honest, which was great," he said.

So can anybody smash the Federer-Nadal duopoly at the tournament featuring the season's top players? The most likely candidate appears to be Zverev, who has rocketed up the rankings to number three after an outstanding season in which he has won five tournaments, including two Masters titles.

"Winning two Masters series is special for any player in an era where the top events have been monopolised by the 'Big Four'," said the German, who is in the 'Boris Becker' group along with Federer, former US Open winner Marin Cilic and Jack Sock.

He added that the younger generation was making a push towards the top of the game to challenge the existing powers. The singles tournament, which runs until November 19, begins with Thiem taking on Dimitrov. The top two in each group qualify for the semi-finals.