Kei Nishikori of Japan celebrates his victory over Novak Djokovic in the semifinals of the U.S. Open Saturday in New York. Djokovic, of Serbia, was the top seed. (Stan Honda/Getty Images)

The final piece of Roger Federer’s best-case dream-draw fell into place just a couple of hours before he took the court late Saturday afternoon at the U.S. Open. In the bowels of Arthur Ashe Stadium, he watched his rival Novak Djokovic wilt in the heat in the day’s first semifinal. A short rainstorm between matches gave the thought time to sink in: Never before had Federer’s path to elusive Grand Slam title No. 18 been more favorable. And probably never will it be again.

The only thing Federer had to do to advance to Monday’s final, with its prospect of getting his hands on that silver trophy by early evening for the first time in six years, was to beat a man he had never lost to. It was right in front of him. It was that easy: He could win the U.S. Open this year without having to face Djokovic, Rafael Nadal or Andy Murray.

But Croatia’s Marin Cilic, who had never beaten him in five tries — winning a total of just two sets in those losses — nearly chased Federer off the court with his booming serve, winning in straight sets, 6-3, 6-4, 6-4, to set up a U.S. Open final against Japan’s Kei Nishikori that no one in the sport could have seen coming. By shocking Djokovic, 6-4, 1-6, 7-6 (7-4), 6-3, Nishikori became the first Asian-born man to reach a Grand Slam final.

It is also a new-blood milestone that will test the memory of even the most passionate tennis fan: the last Grand Slam final that didn’t include at least one of the Djokovic-Federer-Nadal triumvirate that has dominated the sport for a decade-plus was the 2005 Australian Open — a staggering 39 slams ago — in which Marat Safin beat Lleyton Hewitt.

Two days after Federer, 33, required five sets to survive Gael Monfils in the quarters, he never mounted a serious challenge against the 6-foot-6 Cilic, whose thin Grand Slam résumé — one semifinal appearances in 28 tries entering this Open — made him an unlikely candidate to put away the man who is arguably the greatest player of all time. But Cilic, 25, won 87 percent of his first-serve points, showing a consistency that usually has eluded him.

“He served big. He served close to the lines,” Federer said. “When you do that, there’s only so much [an opponent] can do. . . . I couldn’t hang with him long enough to create some doubts in his mind.”

For Federer, four more precious months will tick off his athletic body-clock before the 2015 Australian Open, his prospects for an 18th Grand Slam singles title growing ever so dimmer. He has just one major title, the 2012 Wimbledon, in his last 19 tries.

If he couldn’t win here — with Nadal sidelined by injury and Murray and Djokovic, on the other side of the draw, both bowing out before the final — it is fair to wonder whether he ever can again. Perhaps not wise to wonder — he always will be a threat to win Wimbledon, after all, where this year he lost in five gripping sets to Djokovic — but fair.

“I’ll give it a go again in Australia,” Federer said of the calendar’s next Grand Slam. “I hope to, you know, get another chance at it. I can’t do more than try really hard, which I’m doing.”

In the first semifinal, Nishikori’s thorough dismantling of Djokovic may have been the biggest upset in recent men’s tennis history, though you could argue that distinction lasted only a couple of hours.

There are humans across this great earth capable of vanquishing Djokovic on a hard court, but those individuals are few and far between, and both history and common sense would have told you that Nishikori, as nice a U.S. Open as he was having, was not among them.

A 5-foot-10 speedster from a nation — in fact, a continent — that had never sent a man to a Grand Slam final, Nishikori, 24, nearly withdrew from the Open in the weeks leading up because of pain in his right foot, which required surgery last month to remove a cyst. Instead, he played — bettors could have had him at 280-1 odds to win the whole thing — and had to slog through 10 sets and nearly nine grueling hours of tennis, combined in advancing through the round of 16 and the quarters. That brought him to Saturday, to Djokovic, a man who had lost just one set the entire tournament.

It was to the great shock and amazement of all, then — including, admittedly, Nishikori — that as heat-thunder rumbled in the distance, near the swollen middle of a 96-degree afternoon, Nishikori watched one final Djokovic forehand skid by his feet long, then dropped his racket and pumped both his arms.

The match pivoted on a gripping third set that Djokovic ultimately gave away with a sudden meltdown during the tiebreak, in which he committed four unforced errors and a double fault. This came after a second set in which he had dominated.

At the time, that second set felt like a massive machine coming to life. Afterward, it looked like a sad aberration. Following his third-set flame-out, Djokovic failed to hold serve in the opening game of the fourth set, and the upset was on. On the final stat-sheet, Djokovic won more total points and had more winners and fewer unforced errors than Nishikori, but that was of little consolation.

“What can I say?” said Djokovic, a seven-time Grand Slam champion. “Other than that second set, my game was not even close to what I wanted it to be. A lot of unforced errors. A lot of short balls. Just wasn’t myself.”

And that about sums it up. Saturday was a day when nobody was themselves. Not the underdogs who played out of their minds. Not the superstars who rarely make such grievous missteps.

As a result, on Monday, the U.S. Open men’s final will not be its usual self either.