Roger Federer vs. Novak Djokovic: Score, Reaction from 2015 ATP World Finals

Novak Djokovic isn't out at the 2015 ATP World Finals, but Roger Federer offered a crushing blow to his chances of winning his fourth straight year-end event Tuesday.

Poised from the outset, Federer earned a 7-5, 6-2 triumph to become the first semifinalist out of the Stan Smith group. The third-seeded Swiss needed only an hour and 17 minutes to pull off the upset, battling through a tough first set before dominating the second.

"It's always a thrill to come out here, even more so against a great player like Novak," Federer said, per Piers Newbery of BBC Sport. "It's a special night for tennis, and a cool atmosphere."

Djokovic, who will still have a chance to advance if he defeats Tomas Berdych in his final round-robin match, had not lost at the ATP World Finals since 2011. He'd rampaged through the eight-man field each of the last three years and used last year's event as a springboard to a brilliant 2015. The world No. 1 has captured 10 calendar-year titles and came into Tuesday with a 23-match winning streak.   

Tim Ireland/Associated Press

“You have those days as well when you're not feeling your best—not even close to the best," Djokovic said, per the ATP. “Credit to Roger for mixing up the pace, giving me always a different ball. He used the slice and spin very wisely. He served very efficiently. I made a lot of unforced errors [22]. Just handed him the win, especially in the second set."

Federer nailed six aces and won three-quarters of his first-serve points. Djokovic, meanwhile, could barely find his top form. He took just 51 percent of his first-serve points and won only a quarter of them in the second set. It was strange, but Djokovic actually fared far better when he took some speed off the ball on his second serve.

“But, again, that's sport,” Djokovic said, per the ATP. “Sometimes these kind of matches, these kind of days happen. The good thing is that it's a round-robin system, so I still have an opportunity to reach semis.”

Newbery noted Federer's return ability:

The win for Federer helps put him back ahead of Djokovic in their head-to-head series. The pair has played 43 times on tour, with Federer coming out the winner in 22 of those matches. Djokovic had closed the gap in recent years, including a 4-2 record in 2015 heading into Tuesday.  

"The tactic was to mix it up," Federer said, per Newbery. "I have to against Djokovic. I hit it well out wide today. I knew it was an important serve to get Novak off the court."

Given the way the pair has been playing, it wouldn't be a surprise to see the two go head-to-head again.

Rafael Nadal and Andy Murray look like the two favorites in the Ilie Nastase group, and Federer and Djokovic have both had quite a bit of success against their contemporaries this year. Murray is the crowd favorite, and Nadal is looking to rebound after an injury-riddled season. But the likelihood here of a Federer-Djokovic rematch seems high.

After the way Federer dispatched of him Tuesday, look for Djokovic to try to make it one for the ages.