And Federer delivered in front of his adoring public at the O2 Arena in London on Tuesday -- the Swiss is perennially the crowd favorite when he takes to the court -- by beating tennis' most dominant men's player this season 7-5 6-2 to book a spot in the semifinals of the tournament he has won a record six times.
Federer also disposed of Djokovic's 15-match winning streak at the year-end championships and not since 2012 had the 28-year-old lost indoors.
Oh, Federer was the last player to get the better of Djokovic this season, in Cincinnati in August. Djokovic made up for that, however, by prevailing in their tussle in the U.S. Open final
to grab a third major in 2015 and 10th overall.
Federer lost a mere 15 points on serve in 10 service games Tuesday, evidence that he served well.
But Djokovic, one of the game's best ever returners, was clearly out of sorts, and on a court that Federer described Sunday as slow -- which should have boosted Djokovic's chances on returns.
Indeed Djokovic was nothing like the player of the last two months, hitting 12 winners and committing 22 unforced errors as he slipped to 21-22 in his rivalry with Federer.
Djokovic particularly struggled in the second set, at one stage in the seventh game sending an inside-out forehand well outside the tram lines.
"I made a lot of, lot of unforced errors," Djokovic told reporters. "Just handed him the win, especially in the second set.
"I didn't have this follow through on my shots, no depth, honestly, many mistakes," he added. "Basically if you play like this against Roger, you have no chance."
It might seem unjust considering his glittering season -- Djokovic fell to 79-6 -- but he could be eliminated in the group stage on Thursday when he faces Tomas Berdych.
With all his brilliance, and his own good season, it's easy to forget that Federer is 34. His advanced tennis years haven't slowed down the record 17-time men's grand slam champion in 2015.
"I know it's a big win," said Federer. "Could help me for what's to come in this tournament, or at least for the next match it's going to give me a lot of confidence.
"It's going to help me for next year. It's always good beating Novak or any top player moving forward."
Federer saved a break point to begin the affair and would pounce at 6-5. On a second set point, Djokovic failed to put away a smash, keeping Federer in the point. Federer transitioned, approached the net and executed a perfect half-volley drop shot to seize the initiative.
A rattled Djokovic fell behind 2-0 in the second, but when he broke back for 1-2, some surely were expecting a sustained charge from him. It never materialized.
Still, Federer backed Djokovic to recover.
"To me Novak is still the favorite of the tournament," he said. "Heavy favorite in the Berdych match, in my opinion. Then he should make his way to the semis somehow ... and he'll be the favorite in that probably, too, with the year he's had.
"He's far from gone. The way I know Novak, he's going to find a way to be tougher to beat from now on. It was important to take advantage of the fact he was not playing so well today."
Nishikori on the board
Earlier Tuesday, Kei Nishikori beat Berdych 7-5 3-6 6-3 in two hours, 23 minutes to give the tournament its first three-set match of 2015 after four less than exciting encounters on Sunday and Monday.
Nishikori was crushed by Djokovic to begin the tournament after entering London uncertain following injuries to his shoulder and stomach -- though the 25-year-old hasn't been the only one swept aside by Djokovic this season.
The 2014 U.S. Open finalist from Japan must have been pleased to see Berdych on the other side of the net, since he had beaten the Czech in three of their four previous matches. Further, Berdych had lost eight of his previous 11 matches at the season-ending showpiece.
Nineteen forehand mistakes, plus a low first-serve percentage of 50, contributed to the 30-year-old's defeat. He will rue missing a break point at 3-3 in the third, when Nishikori appeared to play with a broken string, making the ball difficult to control. Yet Berdych erred on a backhand with Nishikori stranded.
Nishikori improved to 1-1 in Group Stan Smith as he bids to make the semifinals for the second straight year, while Berdych -- who hasn't reached the semis since 2011 -- fell to 0-2. Nishikori meets Federer on Thursday.