• Nadal due to meet David Goffin on Monday but ‘can’t predict’ if he will play • Roger Federer says he may never appear at French Open again

Rafael Nadal and Roger Federer, whose parallel dominance has defined modern tennis, will bring their triumphant seasons to a curiously uncertain conclusion in London next week.

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Nadal, 31 and world No1 for the fourth time, hinted he could yet withdraw from the ATP World Tour Finals at the O 2 Arena, where he is scheduled to open his campaign on Monday night against David Goffin. Meanwhile Federer, 36, revealed on Friday he may never play again on clay, where Nadal has always had his number (not to mention that of every player in the game). He plays on Sunday afternoon against the debutant Jack Sock.

Asked about the knee injury that stopped his progress in Paris a week ago and threatened to bring his 2017 campaign to a premature close, Nadal shrugged and raised his famous eyebrow. “I don’t have to make a decision [yet],” he said. “I’m going to play – that’s what my feeling is today. I can’t predict what might happen in the next couple of days, but my feeling is that I’m here to play and to try my best.”

So, a bit of wriggle room there for the oldest year-end No1 in ATP ranking history who has always resented the fact Federer, a six-time winner here, is playing on his favourite surface. Nadal has pulled out of five of 13 World Tour finals: in 2005, 2008, 2012, 2014 and again last year.

Federer said it would be a shame if Nadal could not play. The Swiss, looking relaxed and fit after his own minor break, observed: “He’s the No1 player in the world, had the best season of us all and is a superstar of our game. So naturally it would be a blow. If he’s here, to me that’s a good sign.”

As for his own long-term future, it is becoming obvious that clay will play a peripheral part in his plans in the concluding stages of his career. He has not played at Roland Garros since 2015 – but to declare he would never go back would not go down well with his sponsors there.

“I’m considering all options,” he said. “I just want to make sure of the first steps. Even though it’s not connected, I always see the season sort of until Miami, and then you connect it all the way through Wimbledon, see what the whole clay-court season would look like, and going into the grass-court season.

“I’m talking about that with my team right now to see what they think because they were a big part of me taking the decision at the end not to play the clay court season last year.

“We were on clay for a few days and they said: ‘Think about it wisely because remember the year before you had problems with your knee on the clay; maybe this year is not the right year.’

“That’s when I said: ‘OK, maybe it is the right decision not to push it because it’s been a great season so far.’ So for next year, everything’s back on the table. Anything’s possible.

“Of course I’m hoping to play. I’ve just got to be very cautious about my decision-making. Is it better to make a decision early, or is it better to wait and see what happens during the beginning of the season? Because there is no guarantee I’m going to play as well as I did this year at the beginning of next year. I’m going to have to take everything into consideration. I’ll let you guys know, I guess.”

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And guess is all anyone can do about either of them. They are ruling their sport again, against all expectations, and making the most of the injuries that forced Andy Murray, Novak Djokovic and Stan Wawrinka, their nearest rivals, to quit the season early.

However, that is not why they arrived again at the summit of the rankings. They are there on merit, sharing the four 2017 slams equally between them, and fans will flock to watch their contrasting but complementary skills – perhaps for the last time here, perhaps not.

Asked if he relishes a chance to play Federer here to erase the memories of losing to him five times in a row, Nadal said: “I don’t need to beat Roger to be in good confidence. It would be great to finish the year playing against him again, to give me another chance.

“But you cannot forget that we played all the time on surfaces that he likes more than me. [I have to] just accept that and find different ways to approach the match. If that happens and I am healthy enough, I hope to have my chances.”