LONDON -- Roger Federer and Novak Djokovic prevailed on Thursday to join Rafael Nadal in what is shaping up to be a fantastic four this weekend at the World Tour Finals.
Federer was already assured of a 13th semifinal spot in 14 years at the season finale, so it was perhaps only natural his level would drop off as he squeaked past an inspired Kei Nishikori 7-5, 4-6, 6-4 in a match full of twists and turns.
The victory confirmed Federer's progress as group leader with Djokovic simply having to avoid losing in straight sets against Tomas Berdych to qualify in second spot.
The world No.1 only needed 42 minutes to make sure of that, breaking Berdych for the second time to win the opening set before wrapping up a 6-3, 7-5 victory.
He now renews his rivalry with Nadal, who leads their tightly-contested head-to-head 23-22, though Djokovic has won all three meetings between them this season including the Beijing final and twice on clay in the Monte Carlo semis and Roland Garros quarterfinals.
"It's a fact that he's [Nadal] been playing better and he's been raising his level ever since U.S. Open," said Djokovic in his post-match press conference.
"Already in China, playing finals in Beijing, semifinals in Shanghai, you can feel that he's starting to get comfortable on the court, start to play better. The first couple of matches he played here against [Stan] Wawrinka and [Andy] Murray showed that he feels more confident, that he's starting to miss less, serve efficiently, use his forehand much better.
"I know that, I've been watching. I know what is expecting me. We played so many times. I'm going to get ready for that one and hopefully I'll be able to play at my best."
For Djokovic, it was important to bounce back from Tuesday's shock loss to Federer, which snapped streaks of 38 consecutive indoor wins, 23 overall since August 23 and 15 straight at the World Tour Finals.
The world No.1 is still chasing a record fourth consecutive title here, though, and becomes the first player to win 80 matches in a single season since Nadal (82-11) in 2008.
To his credit, Berdych managed to break the Djokovic serve once in both sets but stopping the Serb completely was always likely to be a tough ask -- Djokovic now leads their rivalry 21-2, with 18 consecutive wins on hard courts and eight in a row overall.
Federer will meet either Andy Murray or Stan Wawrinka in Saturday's other semfinal, depending on who wins their final round-robin clash on Friday.
"At this point everything is tough and fun at the same time because it's cool playing those kind of guys in the semis of the World Tour Finals," Federer said.
"This is what it's about. This is where I wanted to be. I'm going to be happy to be there myself, seeing them battle it out tomorrow."
Nishikori knew he had to go for it to pull off a shock and give himself a chance of reaching the last four, with Federer having to break him three times to win the first set.
The Swiss then surrendered a 4-1 lead in the second set as Nishikori ensured a decider, and let the same advantage slip in the third set before pouncing to take his first match point, closing out a marathon contest in two hours and 10 minutes.
In the end Nishikori was, perhaps, unlucky to lose, and received a standing ovation form those remaining in the O2 Arena as he walked off court, his tournament over.
He deserved it -- Federer was broken five times overall, having dropped serve just once in his wins over Tomas Berdych and Djokovic.
Did knowing he was already through to the last four affect Federer's performance? "Maybe. I really have no clue," he said in his post-match press conference.
"The good thing is that I was through. It relaxes your nerves. I've been in this position before so I know how to handle it."