Novak Djokovic will take on Stanislas Wawrinka in the other semifinal Sunday at the year-end tournament. Djokovic beat Richard Gasquet 7-6 (6), 4-6, 6-3 in his final round-robin match to extend his winning streak to 20 matches since his loss to Nadal in the US Open final in September.
Federer and del Potro needed a win their final round-robin match to progress from Group B. They treated the crowd to a tense and entertaining match, with Federer trailing in all three sets.
Federer has the chance to prove that despite his aging legs, aching back and recent lack of consistency he can still beat his rival Nadal.
"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 said. "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."
He's reached the semifinals 11 of 12 times at the season-ending tournament featuring the top eight players. The 17-time Grand Slam winner has struggled this season and the elite event is his last chance to win a big title in 2013.
Federer has just four victories against top-10 opponents in 2013 and has not beaten Nadal since 2012. The top-ranked Spaniard leads 21-10 in their head-to-head record, but Federer is 4-0 on indoor hard courts, having secured all four wins at the ATP Finals.
Nadal, who returned to tennis in February after a seven-month layoff because of a knee injury, clinched the year-end No. 1 spot this week. This season, he won the French Open, the US Open and qualified for the semifinals unbeaten In London.
"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," Federer said. "Hopefully, it's going to match up good for me. If not, he deserves it. He's had a wonderful season."
Federer only won the grass-court event in June in Halle, Germany. He regularly lost to unseeded opponents and only recently secured his qualification for the finals in London.
"The goal is always to win every tournament. But I think I had to be slightly more realistic in recent weeks and months," Federer said. "I knew that sometimes just getting a few rounds into the bag was positive."
Federer had a slow start, struggling to return del Potro's big serve in the first set and committing seven forehand mistakes as his rival opened up a 5-1 lead.
Federer came back and broke del Potro at love as he served for the set, but the Argentine former finalist saved two break points in his next service game before closing it out.
Capitalizing on his opponent's errors, del Potro broke again in the third game of the second set when Federer's backhand hit the net.
The Argentine looked set for an easy win but failed to build on the momentum, double-faulting as Federer tied it at 3-3.
"I think I got two chances to win the match," del Potro said. "But in the end you have to be focused and to find the winners. I made the mistakes, and he was there really focused to take the chance."
The Bryans have already clinched the year-end No. 1 spot for a fifth straight year and record ninth time. They have won 11 titles this season, including the Australian Open, French Open and Wimbledon.