Rafael Nadal and Novak Djokovic ready for ATP World Tour Finals summit

Rafael Nadal, right, shakes hands with the beaten Roger Federer after their ATP World Tour Finals semi-final. Photograph: Glyn Kirk/AFP/Getty Images

Determining who is the best tennis player in the world is not as straightforward as it might have been in the days of fluctuating rather than predictable fortunes. While Rafael Nadal was missing from the 2012ATP World Tour Finals because his knees had collapsed at Wimbledon five months earlier, he was still regarded as a silent member of the Big Four that otherwise included Novak Djokovic, Roger Federer and Andy Murray.

Now, after a seven-month break and the most compelling comeback since that of André Agassi, Nadal is about to test his credentials against Djokovic, whom he displaced as world No1 a month ago. Their 2013 final at the O2 Arena in Greenwich on Monday night promises to be a memorable and perhaps definitive occasion.

Over the past 12 months meanwhile,Nadal's long-time sparring partner, Federer, has slipped to No7 in the rankings, although he continues to harbour ambitions of returning to the summit of the game at 32. In the semi-finals on Sunday, however, the Spaniard beat him for the 22nd time in 32 contests, 7-5, 6-3 – his eighth win in their past 10 meetings and his first indoor success against him in five attempts.

Djokovic, near his majestic best, later handed Federer's compatriot, Stanislas Wawrinka, a 6-3, 6-3 beating – although Wawrinka maintained: "For me Roger is the best player ever." NeverthelessFederer has become peripheral and Djokovic will go into the final a marginal favourite against Nadal, whose capacity for shocking his peers remains unsurpassed.

Djokovic said: "This is probably the most competitive tournament we have after grand slams and we both want to crown the season in the best possible way and end it with a title."

If the consensus is that Nadal and Federer are the two best players of all time and Nadal and Djokovic rule the modern game, the man from Mallorca would not be drawn. That judgment, he insisted after winning the first semi-final in an hour and 19 minutes, was not only for others to make but should be reached in a wider historical context, pointing specifically to the achievements of Rod Laver.

Five years younger than Federer at 27 and four titles shy of his 17 majors, Nadal argued: "It will be very difficult for anyone to improve his numbers. I never saw Rod Laver play but I know his history. He was away for a few years because he became a professional but he was able to win all four [majors] before and all four after. Today the question is not about me or Roger, more about Rod Laver or Roger. When I finish my career, we'll see where I am in the history of tennis."

Federer, aggressive and dominant in the early exchanges but unable to sustain the quality of his shot-making on weary legs in the second set, said he had "no regrets" after losing the last match of his most disappointing season since 2002, although he would not declare if he could add to his grand slam tally in 2014, when he will turn 33.

It was always a touch-and-go proposition whether or not he could win his seventh end-of-year title here. Briefly in the first set that looked within his scope but the match turned on a couple of missed chances in the fifth game, then again in the 10th. The momentum was with him then, as he pinned Nadal deep and hit his ground strokes with dazzling force and precision. But once Nadal sealed the set at the second time of asking, he began to play more freely – and was mightily relieved when Federer's concluding backhand volley went long.

Nadal is reaching for his 11th title in 14 finals this year and has won 75 of 81 matches. For Federer 2013 ended better than it might have done.

After playing well in the Australian Open – he cites his semi-final defeat by Murray as among his best performances – Federer suffered inexplicable losses, notably to the hit-and-hope serve-volleyer Sergiy Stakhovsky at Wimbledon, before recovering in Asia and Europe in recent weeks. In this tournament he has played some delightful tennis and came close to recapturing his old aura.

"If it's not world No 1, then I'm not that interested in [rankings]," he said, "even though it would be nice to stay in the top four, top eight for seeding purposes more than anything else. It's really a tour of winners." Having won 923 matches and 77 titles in his career, Federer knows what he is talking about.