So much for the first US Open match-up between Roger Federer and Rafael Nadal.

Federer failed to live up to his end of the bargain.

Hours after Nadal did his part with an easy-as-can-be victory to get to the semi-finals at Flushing Meadows, Federer was unable to join him for what would have been the most-anticipated showdown of the entire two weeks, wasting chances to take control and missing shots he normally makes in a 7-5 3-6 7-6 (8) 6-4 loss to 2009 champion Juan Martin del Potro on Wednesday night.

"He came up with the goods when he needed to," Federer said, "and I helped him a little bit sometimes, too, maybe."

Federer described himself as too "edgy" and said "that little magic was missing."

He also insisted that, unlike just about everyone else, he did not spend any time at all pondering a possible match-up with Nadal at the only grand slam tournament where they've never met. Their wonderful rivalry has included 37 matches, and at least two finals at each of the other majors.

"I had struggled too much throughout the tournament," Federer said, "to think too far ahead."

Given that he is 36, and Nadal is 31, perhaps it is time for the tennis world to concede that Roger v Rafa at the US Open just will never happen.

This is the sixth occasion where they were a round away from playing in New York, but one or the other lost — including eight years ago, when del Potro beat Nadal in the semi-finals, then Federer in the final to end the Swiss star's run of five consecutive titles at the tournament.

This time, Federer entered the quarter-finals 18-0 in grand slam play this season, including titles at the Australian Open and Wimbledon to raise his record count to 19 major championships.

But he tweaked his back at a tournament last month, curtailing his preparation for the US Open, and he was not quite at his best for stretches. He needed five sets to win each of his first two matches — and on Wednesday he succumbed to del Potro's familiar formula of massive forehands and booming serves.

Before the US Open began, Nadal was honest as can be when asked whether he hoped to face Federer. The answer, the No. 1-seeded Nadal said earnestly, was no — because he'd rather go up against someone easier to beat.

Well, as it turns out, he'll face the 24th-seeded del Potro today.