Six-time ATP World Tour Champion Roger Federer has had to come from a set down to beat Juan Martin Del Potro to reach the semi-finals of the event in London.
Federer rallied past Juan Martin del Potro 4-6, 7-6 (2), 7-5 on Saturday to reach the semifinals of the ATP World Tour Finals for an 11th time and set up a clash against top-ranked Rafael 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 and del Potro needed a win in their final round robin match to progress from Group B and treated the crowd to a tense and entertaining back-and-forth match. Federer trailed in all three sets before coming out on top.
"I don't get a day off, I've got to back it up and be ready to go again tomorrow. I'll try to make his life a bit difficult," Federer said of Nadal.
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."
Earlier, Wawrinka beat David Ferrer on Friday to clinch his semi-final spot.
"It's certainly a dream season,'' said the seventh-seeded Wawrinka after rallying to beat David Ferrer 6-7 (3), 6-4, 6-1 in his final round robin match.
"I started as the world No. 17, I then reached my first quarters at the French Open, I made it to US Open semis and I'm now doing well at the Finals. It's magic.''
Wawrinka still had a nervous wait as he needed Nadal to beat Thomas Berdych in the late match to advance.
The top-ranked Spaniard had already clinched a place in the semis but did not falter, winning 6-4, 1-6, 6-3 to finish the group stage with a 3-0 record.
Nadal's decisive break
Nadal just needed to claim a set against Berdych to be guaranteed of first place in Group A and lost his grip on the match in the second set, dropping his serve twice. However, Nadal made the decisive break in the eighth game of the decider after Berdych double-faulted twice.
"I wish him a good holiday and good luck for the Davis Cup,'' Nadal said of Berdych, who will be playing in next week's Davis Cup final between defending champion Czech Republic and Novak Djokovic's Serbia in Belgrade.
"In the second set he played great, it was very difficult to stop him.''
Defending champion Novak Djokovic has won Group B - meaning he'll play Wawrinka in the last four - and the winner of Saturday's match between Juan Martin del Potro and Roger Federer will face Nadal in the semi-finals.