Roger Federer takes underdog role for Rafael Nadal showdown

Roger Federer insists he is happy to be labelled as the underdog in the latest instalment of his epic rivalry with Rafael Nadal after booking a semi-final showdown with the Spaniard at the ATP World Tour Finals.

Federer defeated Argentine fourth seed Juan Martin del Potro 4-6, 7-6 (7/2), 7-5 on Saturday to ensure a second place finish in Group B and a 32nd career meeting with world number one Nadal.

In the other semi, defending champion Novak Djokovic, who finished Group B with a 100 percent record after a 7-6 (7/5), 4-6, 6-3 win over Richard Gasquet on Saturday, takes on Switzerland's Stanislas Wawrinka.

Federer, a six-time Tour Finals champion, is desperate to finish one of the worst campaigns of his illustrious career with yet another triumph in the prestigious season-ending event at London's O2 Arena.

The 32-year-old knows Nadal goes into the tie as favourite after capping a remarkable return from a serious knee injury by winning the French and US Opens, as well as eight other titles, en route to securing the first place in the year-end rankings.

History also favours Nadal, who leads their head-to-head 21-10 and has won 13 of their last 17 matches, including all three this year.

But Federer can take comfort from his record of winning all four of his indoor clashes with Nadal, including the 2010 final at the O2 Arena.

"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," Federer said.

"Maybe that free swinging is what I need to do a little bit more tomorrow (Sunday).

"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."

For most of Saturday's winner-takes-all clash with del Potro it looked like it would be the 2009 US Open champion who would be going through to meet Nadal.

Del Potro took the first set with a dominant display and moved a break ahead in the second, only to let Federer back in and then surrender the tie-break.

Even then, it was del Potro who was in control again with a 3-0 lead in the final set before Federer finally produced some of his best tennis to roar back and seal the victory.

Asked how he felt at 3-0 down in the decider, Federer said: "I fought back the whole match. I was thinking, 'Here we go again'.

"I was really getting angry at myself, how could I play a loose game, why is this happening?

Once on even terms, I was able to play a little bit more freely.

"It was a great finish. I was very happy. To get the victory was a great feeling."

World number two Djokovic, bidding for a third Tour Finals title, will go into his meeting with Wawrinka on a 20-match winning run after defeating French eighth seed Gasquet in a dead rubber.

The Serb, who has racked up 70 wins for the third straight year, had already qualified for the last four as group winner, but he showed no signs of letting up against Gasquet, who had been eliminated after losing his first two matches.

Djokovic has won his last 13 matches against Wawrinka, including a pair of thrilling clashes at the Australian and US Opens this year.

But Wawrinka has made the last four on his debut and Federer said of his compatriot: "I don't know if I'm more excited that I won or we're both in the semis. It's historic."