NEW YORK -- This was the moment when all the elements coalesced, when the aging champion suddenly looked old.
Roger Federer's fifth-set shot took Novak Djokovic five yards past the doubles alley and, as he drifted toward the net, he might have been thinking the point was over. But the astonishingly flexible Djokovic reached the ball and hit a blooper back over the net. Federer, with the whole court open, dumped a forehand into the net. One of the greatest shots in the history of tennis looked fragile, even weak.
Djokovic was unfortunate to come along when he did; he'd own a few more Grand Slam titles if Federer and Rafael Nadal weren't winning them all. And yet, the 23-year-old Serb has played with remarkable consistency this year, reaching at least the quarterfinals in all four majors for the first time.
But until Saturday he almost always seemed to bump into a glass ceiling.
The last four times he'd played in Grand Slam semifinals -- a lifetime achievement for most players -- he lost. On three of those occasions, the winner was Federer.
But Federer, at 29, has come back to the field this year.
He fell in the quarterfinals at Roland Garros and Wimbledon and has started losing to players that never challenged him. He isn't quite as swift as he used to be; his dazzling game is now touched by periods of awkwardness.
Federer had been to six straight finals here, a towering achievement. But last year, after winning two of the first three sets against Juan Martin del Potro, Federer uncharacteristically shrank from the moment. A man seven years younger lifted the sterling trophy.
Similarly, on this breezy day at Arthur Ashe Stadium, Fedrerer did not quite seem up to speed. Djokovic and his forehand slashed their way through two massive match points and defeated Federer 5-7, 6-1, 5-7, 6-2, 7-5.
"I am very proud of myself," Djokovic said later. "There are a lot of emotions involved. Of course, I was too exhausted to show them in the end. But it's been a fantastic semifinal."
He will play No. 1 seed Nadal in Sunday afternoon's U.S. Open final. Earlier, Nadal defeated Mikhail Youzhny 6-2, 6-3, 6-4.
When it was over, after Federer lost three break points to force a fifth-set tiebreaker, Djokovic stood dead still in the rattle and hum of a 23,000-person standing ovation. His eyes were wide, as if an electric current was running through him.
Maybe it was. How did he beat Federer?
"I don't know," he said in his on-court interview. "It's really hard to describe the feelings I have right now. Ten minutes ago I was a point from losing this match.
"[On match point] I was just closing my eyes and hitting the forehand as fast as I could. If it goes in, it goes in. It's one of those matches that you will always remember your whole career."
The fifth set was sensational, and its 68 minutes alone could probably win an Emmy for best drama.
Federer, a five-time U.S. Open champion, tried to be aggressive, to hit difficult shots from improbable positions, perhaps knowing he could not win a baseline battle with Djokovic, who is younger, stronger and plays spectacular defense.
"I lost a couple more with match points this year, so they all pretty much feel the same," Federer said. "They feel somewhat empty at the end because you have tried everything, and maybe it was luck. Maybe it was he played well. Maybe you didn't pick the right shot; maybe he did, you know."
Federer's hasty, over-extended game produced 66 unforced errors and only 48 winners -- numbers that are usually reversed.
"Can't turn back time, but, look, he obviously had to come up with a couple of good shots on match point, so I don't feel I have that many regrets in that regard," Federer said. "Yeah, it wasn't the final, so I'm not as disappointed as if it would have been the final. That's the only positive news to enjoy anything out of it."
Heading into the match, Djokovic was only second man in 20 years to reach four consecutive semifinals here -- Federer, of course, is the other.
Djokovic was in solid position to take the first set, serving at 4-2, when he started to back up. A double fault at 30-all gave Federer a break point, which he consolidated with a nifty inside-out backhand and an overhead. Federer served it out and, when Djokovic -- looking to his box in frustration -- double-faulted to open the second set, it looked like it might be over.
That was when Djokovic won six of the next seven games. One -- the fewest games Federer has won in a set here since 2001, when Andre Agassi beat him in straight sets.
Serving to force a third-set tiebreaker, Djokovic went AWOL again. Two errant forehands gave Federer the game at love -- and the set with it. And then Federer, in his tennis version of the rope-a-dope, coasted through the fourth set as he did the second. At one point in the last game he hit a volley straight at Djokovic, practically begging him to hit a winner. Djokovic hit a lob instead, forcing Federer to run it down, even though it was out.
The entire match came down to two junctures. Serving at 4-5, Djokovic blew an overhead and two backhands to create two match points for Federer. On both, Djokovic ripped forehand winners and, later in the game, two more. And then, Federer had three chances to force a tiebreaker. He yanked a forehand wide and he was done.
It was the third time this year that Federer has held match points but gone on to lose.
"I feel like I'm playing well, and I would have deserved to be in the finals tomorrow," Federer said. "It's a tough loss for me, but it's only going to fuel me with more motivation to practice hard and get back to Grand Slam finals, which I haven't been in for the last three Slams."
Djokovic will be looking for his second major title, and before you give Nadal his ninth Grand Slam singles crown, consider that Djokovic is 7-3 against Nadal on hard courts.
What was Djokovic planning for his recovery? Massage. Ice bath. Food?
"No," he said. "Popcorn, watching TV, relaxing. Yeah, I will do anything that comes up to your mind legally, recovery-wise. I will do it. You know, I cannot go to the details too much.
Emotional recovery with my girlfriend, and a couple of things that I cannot talk about. It's not what you think. I know what you're thinking."
Djokovic, who was on the court four hours later than Rafa and 91 minutes longer, was told that rain was in the forecast for Sunday.
"Oh," he said, drawing laughter. "Really? I don't know the rituals how to invite the rain, but an extra day would be great, actually."
Greg Garber is a senior writer for ESPN.com.