“Fear the beard”: Should that be Roger Federer’s new slogan? On Tuesday the usually clean-cut Swiss was showing a hint of facial hair as he walked out to play Novak Djokovic at the ATP World Tour Finals in London. Was this a late-career turn toward the wild side? Federer certainly played with more experimental freedom than he had shown in his last match against the world No. 1, in the U.S. Open final.

Federer kicked his first serve wide, dropped his backhand return short and powered his forehand return down the line, closed the first set with a cheeky half-volley winner—he admitted later that he used the wrong grip—and nearly passed Djokovic from behind his back. It worked to the tune of a surprisingly breezy 7-5, 6-2 win in just 77 minutes. In the process, Federer broke Djokovic’s 23-match win streak and three-year indoor win streak, guaranteed himself a spot in the semifinals of the season-ending championship, and improved his record against Djokovic to 3-4 in 2015. While Djokovic has won three long slogs, Federer has won three lightning-strike straight-setters.

As on as Federer was today, Djokovic was equally, head-scratchingly off. It’s not often that you see him finish with 10 more errors than winners (22 to 12), earn so few break points (he was just 1 of 3), or struggle so mightily with his shot selection. Instead of efficiently controlling the baseline rallies and pushing Federer to go for too much, this time it was Djokovic who was pulling the trigger early, who was going down the line at the wrong times, who was mistiming his returns, who didn’t bend low enough for a Federer slice return one one break point or get enough penetration on his approach shot on another.

Djokovic likes a rhythm, and Federer didn’t give him one, but the Serb also faded down the stretch in a way that he hasn’t all season. Only when he was match-point down at 2-5 did he play with the kind of assertive, heavy shotmaking that we had expected from the start. He saved one match point after a 26-shot rally, but couldn’t do it again.

"You have those days when you're not feeling your best," Djokovic said. "I made a lot of unforced errors."

How significant is this win? Unlike a normal victory, this one doesn’t knock the loser out of the tournament. Djokovic has some work to do when he faces Tomas Berdych two days from now, but would you really bet that he and Federer won’t be meeting again in the final on Sunday?