LONDON -- Roger Federer and Novak Djokovic's rivalry has largely defined this season, so it is perhaps fitting we get to see it one last time before the end of 2015.
The two men will contest their seventh final this year -- Djokovic has won four, including Wimbledon and the U.S. Open -- after Federer saw off the challenge of Stan Wawrinka for the second year running in the semifinals at the O2 Arena.
Federer secured a 7-5, 6-3 victory to to ensure a second meeting with Djokovic, who beat Rafael Nadal 6-3, 6-3, in London this week, having shocked the world No.1 in straight sets in the group stages on Tuesday, snapping his 23-match win streak in the process.
There was a bit of needle after that contest -- which edged Federer 22-21 in front in their head-to-head -- with Djokovic saying he had "handed" Federer the win, before subsequently denying there was any "bad blood" between the two.
"He's played some great tennis since we've played," Federer said in his press conference. "I still believe his confidence is slightly higher than mine with the amount of success he's had this year."
For Federer, it is a chance to make up for having to pull out of last year's final against Djokovic with a back injury.
"[I'm] feeling slightly better than last year," he added. "The press conference wasn't the fun part, I could tell you that, because I knew I was not in good shape. This year obviously it's very different. I feel completely normal. It's a big relief for me. Happy of how the week's gone so far. It's a good feeling this time around."
Before Federer and Wawrinka got under way on Saturday, umpire Ali Nilli had joked, "Let's make this a good clean fight. No blows below the waistline."
There were no fireworks on the sidelines this time, after a reported verbal spat between Wawrinka and Federer's wife Mirka last year at the O2 Arena threatened to sour the two men's relationship going into the Davis Cup final with Switzerland.
Their 2014 semifinal lasted just 12 minutes short of three hours, with Wawrinka failing to take four match points before Federer sealed a 4-6, 7-5, 7-6(6) victory.
This time it was a more routine outing for Federer, who overcame an early loss of service to break twice to take the opening set, before racing into a 3-0 lead in the second set on his way to a comfortable win over a flat, subdued Wawrinka in just 70 minutes.
Federer can take plenty of encouragement into the final after going 4-0 at the tournament, as he chases a record seventh title, which could see him surpass Andy Murray and finish the year as No.2.
Djokovic, speaking before Federer's win over Wawrinka, said: "When he's [Federer] on, it's hard to play him.
"We always play matches with a lot of tension. There is a lot at stake. I know I have to be on top in order to win against him. Hopefully I'll be able, if I play him tomorrow, to play better than I've done a few days ago. But the performance today definitely gives me reason to believe that I'll do that."
It certainly should. Earlier on Saturday, Djokovic ended Nadal's encouraging resurgence at the World Tour Finals with a clinical semifinal win.
The world No.1 ensured his place in a record 15th consecutive final this season, a run dating back to his Australian Open win in January.
Nadal said after the match that the Serb is "almost unbeatable" when in this mood. "I don't feel unbeatable," said Djokovic, "but it does definitely feel great when I'm playing the way I played today."
He added: "It's the best season of my life. Regardless of the outcome of tomorrow's match, the season will still be the best that I had. There is always room for improvement, so that's something that honestly excites me and motivates me to keep on going."
Djokovic has bounced back from his group-stage defeat to Federer with wins against Tomas Berdych in his final round-robin match and Nadal in this semifinal, to take his record for the season to 81-6 -- that is a win rate of over 93 percent.
The Serb will look to crown one of the finest seasons since the Open era began with what would be a record fourth consecutive World Tour Finals title and fifth overall, having also won in Shanghai in 2008.
It is now eight defeats in nine for Nadal against the Serb. Their 46 meetings are the most between any two players during the Open era, with Djokovic tying their head-to-head for the first time at 23-23 after 10 years.
"He was better than me and he deserved to do what he did during the whole season. He played just fantastic. The only thing I can do is congratulate him and just wish not the best of luck for the next year," Nadal said with a smile.
After a season of self-doubt that saw him slip to No.10 in the world in June -- for the first time since 2005 -- Nadal can be proud of the way he has finished it, sweeping his group in London to put together his best ever sequence following a U.S. Open (17-5).
"I need to keep improving things if I want to have chances against Novak next year," Nadal added. "That's what I am going to try. I think I am doing well. I'm happy about finishing the season with that positive energy."