Novak Djokovic came hurtling into the Barclays ATP World Tour Finals on a terrific tear. But there was a little hiccup at the O2 Arena on the way to the Sunday championship final -- a loss to a guy named Federer.
And now, after Saturday's masterful 6-3, 6-3 victory over Rafael Nadal, Djokovic has a chance to punctuate one of the great years in tennis history with his fourth consecutive year-end championship.
And just to ramp up the degree of difficulty, he'll have to do it against a bearded Roger Federer (on ESPN2 & WatchESPN, Sunday, 1 p.m. ET), who later defeated Swiss Davis Cup partner Stan Wawrinka 7-5, 6-3 in a light 70-minute workout.
This was supposed to be last year's final match, but Federer withdrew.
"Regardless of the outcome of tomorrow's match," Djokovic told the press after beating Nadal, "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. I've had a terrific career and I'm very proud obviously with my team to be able to achieve what I have achieved in the last 10 years of professional tennis.
"But I still feel like I want to do more, and I can do more."
The first task is to win one more match in London.
Djokovic has prevailed in 17 of his past 18 on this blue court -- thanks, Roger -- and is attempting to become the first player to win four straight ATP year-end crowns.
A victory would push Djokovic's record to 82-6 (.932) and give him 11 titles, including a record six Masters shields. His mark against top-10 players is an astounding 30-5. Reaching the finals of all four Grand Slam events (winning three of them) was a career first.
There is some nice symmetry working here for the 28-year-old Serb. By beating Rafa, he evened his lifetime series with the Spaniard at 23-23. A win against Federer would, appropriately, level that rivalry at 22-22.
Nadal, who lost all four of his 2015 matches to Djokovic, was asked in his presser what made him so difficult to play.
"Everything," Rafa said immediately. "Hitting amazing. Well, the return always amazing. This year he serving great, I think. And then he is able to play with no mistakes and changing directions so easy, playing so, so long. He's doing everything good.
"He was better than me and he deserved to do what he did during the whole season. He played just fantastic. When somebody's doing like this, just the only thing I can do is congratulate him and just wish not the best of luck for the next year."
Adding some spice to this encounter are Djokovic's remarks after his round-robin 7-5, 6-2 loss earlier this week, that he handed the match to Federer, who seemed to take exception.
"It's not like he played terrible," Federer responded. "I know he can play better. Why did he play that way? I'd like to give myself credit for that, quite honestly."
The 17-time Grand Slam singles champion is 34 years old now, but he's the only player on the planet who's given Djokovic trouble this season. Djokovic has a narrow 4-3 advantage, but Federer has won two of the past three, going back to their final this past summer in Cincinnati.
Federer, serving and volleying, even on some second serves, was uber-aggressive against Wawrinka. On the first point of the final game, his forehand volley was so pure it caused Wawrinka to fall down trying to change direction. Federer will have to take similar chances to beat Djokovic.
With a win on Sunday -- followed by some help from Belgium in next week's Davis Cup final against Andy Murray -- Federer could potentially finish the season at No. 2 for the second straight year.
"When he's on, it's hard to play him," Djokovic said of Federer. "I think he also likes playing me because he has a variety in his game with the slice, he comes to the net. It takes away the time from me. 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."