A point later, with the crowd in hysterics and opponent Novak Djokovic still in shock, the world's top-ranked player closed out the victory, 7-6 (3), 7-5, 7-5, to move one win from his sixth straight U.S. Open title.
OK, who's got next?
Juan Martin del Potro is the lucky guy whose first career Grand Slam final will come Monday against Federer, who made his 17th in the last 18. Earlier in the day, No. 6 Del Potro beat No. 3 Rafael Nadal, 6-2, 6-2, 6-2.
Fourth-seeded Djokovic fought for more than 2½ hours on a day that grew increasingly windy at Arthur Ashe Stadium, hanging with Federer and even grabbing two break points late in the third set to briefly see a glimmer of hope.
Some things, though, there are no answers for, and the winner Federer hit to set up match point was Exhibit A. It's the kind of shot every tennis player has tried - oh, a thousand times or so. The best player in the world practices it, too.
"A lot, actually," he said. "But they never work. That's why, I guess, it was the greatest shot I ever hit in my life."
Ahead 6-5 and 30-0 in the third set, Federer sprinted to the net to return a Djokovic drop shot, then Djokovic finessed a lob over Federer's head that bounced barely inside the baseline. Federer had nothing to lose, of course, so he ran back and hit the circus shot, a ball that lots of players, especially at the highest levels, can get back.
But few can do what Federer did with his - i.e., hit a blazing winner that barely clears the net. Federer jumped and shouted. Djokovic could only stand there and smile. He reached in his pocket to find the ball he'd serve to bring the match to a merciful end - for him, at least.
"You just say, `Well done,"' Djokovic said. "What can you do?"
This was exactly the kind of memory the U.S. Open needed after a weekend filled with rain delays and controversy.
It was, oddly enough, also two points before match point Saturday night when Serena Williams got called for a foot fault on her second serve, then unleashed a profanity-laced tirade at the line judge that cost her a point penalty, which ended the match.
On Sunday, while the Federer match was going on with Jack Nicholson, Paul Simon and Charlize Theron among the celebrities watching from the stands, the U.S. Open hit Williams with a $10,000 fine for that ugly scene.
Federer's shot - that was one-in-a-million.
"I don't want to mention the word luck, but I didn't have it today," Djokovic said. "That's why I'm a little bit disappointed."
Not that there wasn't plenty for him to get demoralized about before "The Shot."
Had that not occurred, the point that would have defined the match - and what it's like to play Federer - came at 5-all in the second set. Djokovic was a sitting duck at the net, yet somehow managed to get five straight reflex volleys back to Federer, who was standing at the service line, teeing off.
Djoko's final volley was a floater and he did what any smart guy would do: He stuck his racket between his legs, turned around and stuck out his rear - the tennis player's version of begging for mercy.
Everyone got a good laugh out of that one, but the mark Federer is leaving on this sport is very serious stuff:
-He's looking for his 41st straight win at Flushing Meadows.
-He's in his 21st Grand Slam final, a record.
-He's reached 22 consecutive Grand Slam semifinals, more than twice as many as any other man.
-He's trying to extend his own record with a 16th Grand Slam title, but first since his twins were born in July.
"Right now, I'm pretty relaxed," Federer said. "We'll see how it goes when the sun comes up. I'd like to keep this going. It'd be great to get my first Grand Slam as a dad."
Earlier, del Potro put on a show that was every bit as efficient as Federer's was spectacular, taking apart Nadal with a flurry of big serves and precise forehands.
Nadal finally acknowledged his strained abdominals were bothering him throughout this tournament, but didn't want that to take away from del Potro's fine effort.
"I'm going to repeat: He played much better than me, and for that reason he beat me," Nadal said.
No arguing that.
The sixth-seeded Argentine - the first from that country to make a U.S. Open final since Guillermo Vilas in 1977 - kept Nadal pinned behind the baseline with a deep, flat forehand and a first serve he mixed at between speeds from the 90s to the 130s.
He is 0-6 lifetime against Federer and hadn't won a set until this year in the French Open semifinals. But, del Potro claims he is seeing the ball very well this week.
"Maybe my green eyes. I don't know," he said. "It's very tough playing against Rafa or Roger. But today I play unbelievable, and that was the key."
On this day, though - and during that one magic moment, especially - it was Federer who had a stranglehold on "unbelievable."