Federer spares a thought for absent Nadal LONDON (Reuters) - Billed as a showdown between the world's top-eight players, the ATP World Tour Finals got off to a spectacular start on Monday with strobe lights, music and victory for home favorite Andy Murray igniting the touchpaper for eight days of action. However, the fourth staging of the season-finale besides the River Thames at London's O2 Arena is missing one of its main attractions with Rafa Nadal's injury-enforced absence taking a little gloss off the tournament. The Mallorcan has appeared in London for the previous three years, reaching the final in 2010 when he lost to old rival Roger Federer, but his recurring knee problems have robbed men's tennis of his ferocious talent for the past six months. While all the talk might be of Murray's rapidly intensifying rivalry with world number one Novak Djokovic, Nadal is being missed, not least by 17-times grand slam champion Federer. Some of the most memorable moments of the Swiss maestro's remarkable career have been with Nadal across the net and he was quick to jog people's memories as he prepared for his opening match against Janko Tipsarevic on Tuesday. "Once a tournament starts, you do unfortunately forget what's happening around you," world number two Federer, bidding for a record seventh end-of-year title and a hat-trick in London, told reporters. You do forget those things a little bit. Knowing that maybe the draw is potentially a little easier, there is no denying that because Rafa is a great champion and I'm sure that he is missed by a lot of the fans, which would have made this tournament maybe even more exciting, but then it gives opportunities to one or two more guys. It's obviously never the same when Rafa doesn't enter a tournament, but from time to time injuries just do happen, and we all wish him the best so he can return next year. Federer, 31, is the elder statesman in the draw, but despite the passing years, his game is still ideally suited to the indoor surface at the 02 Arena and he will be hard to stop if gets off to a flying start against Serb Tipsarevic. Whether or not he retains the trophy, however, Federer acknowledges that Djokovic deserves the end-of-year No.1 ranking having snatched it back from Federer last week. "The real number one, we know who that is going to be, it's going to be Novak, so I think there should not be any debate around that," said Federer who this year moved past Pete Sampras's record 286 weeks spent on top of the ATP rankings. Number one, you don't get there by chance. The rankings are something that show you how you've played over a 365-day period. It might change all over again at the Australian Open, there's no doubt about that, but right now, it's clear. This event is a bonus, obviously, for the top eight players to face off against each other, and try their very best and hopefully finish the season in good style. Federer also agreed that the duels between Murray and Djokovic this year, most memorably in the U.S. Open final, have become the must-see matches for fans after being deprived of the Nadal-Federer blockbusters. "I always knew Andy and Novak were going to stay around for a long time at the very top, so this is just right now it seems they are in their prime and this is when they are supposed to play their best tennis in my opinion," Federer said. Obviously we miss Rafa because he belongs in there, he won a grand slam this year. I would have loved to have seen him again and played him because I don't think I've played him in the last couple of years. Editing by Ed Osmond