Underdog Federer to come out swinging against Nadal LONDON (Reuters) - Roger Federer has never lost to great rival Rafael Nadal under a roof but said he will start as underdog in their semi-final clash at the ATP World Tour Finals on Sunday. The Swiss has endured a lean year, adding only one title to his vast collection, but showed on Saturday that despite the creeping years he is far from a spent force when defeating Juan Martin del Potro to reach the last four at the o2 Arena. Federer trailed throughout an exciting contest on one of the world's most distinctive indoor stages before digging out a 4-6 7-6 (2) 7-5 victory to seal the runners-up spot in Group B. His performance was a mixed bag, fluctuating between sheer brilliance and uncharacteristic errors, but he displayed the spirit of a great champion to pip del Potro at the post. Now, the 32-year-old 17-times grand slam champion believes he can swing freely on Sunday when most people will expect Nadal to confirm his world number one status. "I just have to have the right mindset to give it one last go, maybe play with a little less pressure than I have in previous matches with him," Federer, who has fallen 21-10 behind in overall meetings with the Spaniard. I just feel like I need to look at it more as being an underdog a little bit because of circumstances, because of my year, because of his year. Maybe that free swinging is what I kind of need to do a little bit more tomorrow. Federer's schedule has been a heavy one with a final appearance in Basel, then a semi-final run in Paris before arriving in London for his 12th consecutive appearance at the ATP's annual season-ender. Despite feeling the strain, Federer said he will be ready to fight tooth and nail with Nadal, who had Saturday off. I see the light at the end of the tunnel. There's two more matches and that's it," said Federer, who is looking for a seventh title at the tournament and third since it moved to London in 2009. We know each other that well. He's going to do exactly what he needs to do, I'm going to try to do what I need to do. Hopefully it's going to match up good for me. If not, he deserves it. He's had a wonderful season. We'll see how it goes," added the ever-popular Swiss. An added bonus for Federer is that fellow Swiss and close friend Stanislas Wawrinka is also through to the semi-finals in his first appearance at the tournament. Wawrinka faces defending champion Novak Djokovic in Sunday's second semi-final with the intriguing possibility of an all-Swiss final still a possibility. "I don't know if I'm more excited that I won or we're both in the semis, to be honest," added Federer, who won Olympic doubles gold with Wawrinka in 2008. I'm very happy for Stan. I was hoping to be there, too, make both of us get to the semis. Here we are playing the best two players of the season. I think it's incredibly interesting for the two of us, for Swiss tennis, for the Swiss media. I think we're gonna speak to each other a little bit later and I'll tell him a few things that I know and he'll hopefully tell me a few things he knows this week about the players we're playing. Reporting by Martyn Herman, editing Tony Goodson