The old rivalry will be reignited yet again as Rafael Nadal and Roger Federer battle for a place in the ATP World Tour Finals title round.
Nadal has shown good form in the year-end tournament in London this year, going unbeaten in Group A to seal top place.
That meant a semifinal clash with his good friend Federer, who finished second in Group B behind Novak Djokovic.
Federer had to dig deep to find a place in the last four, battling back from a set down against Juan Martin Del Potro to post a tense 4-6, 7-6 (2), 7-5 victory.
The Swiss great, a 17-time Grand Slam champion, was down 0-3 in the final set, before finding his touch and class to pull off the great escape.
"I was probably slightly angry more than thinking it's going to be over soon," said Federer of trailing in the third set. "It's one of those moments today, because I kind of fought back the whole match - the first, second set. Here we go again.
"I wasn't in many of Juan Martin's service games, so I kind of felt like probably I will get one more chance to break back. It's exactly what happened. Once on even terms, I was able to play a little bit more freely.
"For the first time I was almost feeling like I was kind of in the lead. [It was a] great finish. I was very happy. To get the victory was a great feeling, so I was very happy."
Federer has an outstanding record against Nadal in the ATP World Tour Finals, winning all four of his matches against the Spaniard, who, of course, has a better overall record.
Nadal is yet to win the season-ending tournament, while Federer has held aloft the trophy six times already.
The Spaniard, though, will be buoyed after sealing the year-end world number one ranking, to which he was more than thankful, considering the fact that he missed last year's tournament to a long-term injury.
"One year ago, if you had told me I would have come back to be here... it's been an amazing year," Nadal said. "I can just thank my family. Without their support, this wouldn't have been possible.
"It's been a very emotional year and ending here with so much support at The O2. It's fantastic after what happened last year.
"I never had the feeling [that I am the best player in the world]. Being No. 1 is one of the most difficult things to do in our sport. You have to play well from the beginning to the end and there are lot of fantastic players. For most of this year, I played the best tennis of my career. It was a big surprise for me and I'm very happy."
Where to watch
The match is scheduled for a 7.30 pm IST start with live coverage on Ten Sports and Star Sports HD1.