NEW YORK -- It is an inevitable fact of life: Men of a certain age sometimes have difficulty, uh, finishing. This summer, Roger Federer has come to learn that troubling truism for himself.
Before this year's Wimbledon tournament, he had won every one of his 178 best-of-five Grand Slam matches when taking the first two sets. But in the quarterfinals at the All England Club, up two sets on Frenchman Jo-Wilfried Tsonga, Federer, shockingly, lost the last three sets by identical scores of 6-4.
At the time, it seemed like a random event, just another sign that the 16-time Grand Slam champion was becoming, if only slightly, mortal. It happened here a year ago at the U.S. Open when Federer held two match points against Novak Djokovic in the semifinals and couldn't close the deal.
Now, it's beginning to look like a trend.
Federer throttled Djokovic in the first two sets of their U.S. Open semifinal Saturday but then, inexplicably, shrank from the task. In a mad, manic match that had the crowd at Arthur Ashe in a frenzy, Federer lost 6-7 (7), 4-6, 6-3, 6-2, 7-5.
In an eerie back-to-the-future sequence, Federer held two match points -- and lost them both. Inevitably, it seemed, he dropped the last four games with some sometimes feeble strokes.
"It's awkward trying to explain this loss," Federer said. "It is what it is. It's the obvious reason. He came back. He played well. I didn't play so well at the end. I have only myself to blame.
"I set it up all perfect. I couldn't finish it.
"He snaps one shot, and the whole thing changes. It's strange how it goes. I would be in the finals with a chance to win the title. I have to accept that and move on. It happens sometimes. That's why we all watch sports. We don't know what the outcome is going to be. It's also very cruel and tough sometimes."
To reiterate: The great Federer has now blown two-set leads in back-to-back Grand Slams in what have to be among the most disappointing losses of his career.
It was, surprisingly, only the second two-set comeback of Djokovic's career. He did it to Guillermo Garcia-Lopez at Wimbledon in 2005.
In retrospect, maybe this turbulent, chaotic U.S. Open didn't deserve its first Federer-Nadal final. On Monday, Djokovic will meet the winner of the second semifinal between No. 2 seed Rafael Nadal and No. 4 Andy Murray.
"It was definitely the biggest win of this year," Djokovic said in his on-court interview, "one of the biggest wins of the career, under the circumstances. I switched gears and managed to play much better."
In a match that stretched 3 hours, 51 minutes, Djokovic was better in the big points. Down 5-3 and 40-15 in the fifth set, he erased a match point with a sensational service return, a forceful forehand cross-court winner.
"You feel like he was mentally out of it," Federer said of Djokovic's return. "He gets lucky, the shot goes in and off you go."
Lucky or confident?
"Confidence, are you kidding me?" Federer asked. "Please. Some players grow up and play like that. I never played that way. I believe that [the] hard work's going to pay off.
"This is very hard for me to understand how you can play that shot on match point. Maybe he's been doing that for 20 years. You'll have to ask him."
Djokovic did not exactly disagree.
"If it comes in, it comes in," he said. "It was a risk last year. It was a very similar situation. I was hitting [the] forehand as hard as I could. You're gambling.
"I was lucky today."
On the second match point, Federer's forehand clipped the net and bounced wide.
Djokovic is now 63-2 for the season and still has a chance to produce one of the great seasons in tennis history. A win Monday would deliver his third major of the year.
Federer and Djokovic have played each other 24 times, the third-most prolific rivalry among active players, and you get the idea they probably dream about each other. There are no surprises at this stage. Djokovic, the best returner in the game, gets everything back and Federer, even at the age of 30, is still, sublimely, Federer.
They met for the fifth straight year here, and it is trending poorly for Federer. He won the first three matches against Djokovic, then lost 7-5 in the fifth set a year ago in the semifinals.
Djokovic lost only five points on his serve in the first set, yet Federer somehow managed to force a tiebreaker. He was better, barely, converting his fifth set point with a thunderous forehand winner in what was a splendid 55-minute slice of artistry.
After dropping the second set, Djokovic pulled on a white ball cap and seemed to relax. He started playing like the player who has dominated this year -- and suddenly Federer looked skittish. His fluid movement stopped, and there were times when he looked a little awkward. When Djokovic served out the fourth set, with a little less than three hours on the clock, Federer looked like a man resigned to his fate.
Incredibly, it was the first five-setter for Djokovic since beating Federer a year ago.
The fifth set was marvelous stuff. The nearly 24,000 spectators, pulling hard for a Federer victory, roared throughout. When Djokovic won a spectacular point -- both men were stretched to the limit, hitting balls from the doubles alley -- to level it at 2-all, they jumped to their feet.
The critical break came in the eighth game -- and it was a total meltdown for Djokovic. At love-30, he knocked a second serve long by about three feet. When he sprayed another forehand long, Federer found himself serving for the match.
We know how that worked out.
Federer has been saying for some time that he feels no pressure. His legacy is secure -- as long as he doesn't store his trophies in the storage facility Pete Sampras used.
This semifinal, for many reasons, meant more to Djokovic. There is a big difference, in terms of history, between winning two majors in a single season and winning three. Clearly, Federer relished the idea of preventing Djokovic from doing something he did three times.
And so, there will be no Open era-record sixth U.S. Open title, not this year, maybe ever. This is the first year since 2002 that Federer failed to win a Grand Slam singles title, perhaps the most telling sign that his record-setting total of 16 majors might be the final number.
For Djokovic, it would be a more modest fourth Grand Slam singles title if he beats Nadal or Murray.
"I know both of the players," Djokovic said. "I'm going to sit in the chair and enjoy their match. Hopefully, they're going to play as long as we did."
Greg Garber is a senior writer for ESPN.com.