Roger Federer knocked out of U.S. Open by Tommy Robredo

Another long U.S. Open rain delay may have been an occasion to contemplate why the two best male tennis players of their generation never have played each other on the big Flushing Meadows stage.

But once the weather cleared and Roger Federer had to endure a thorough 7-6 (3), 6-3, 6-4 fourth-round thrashing at the hands of Tommy Robredo, it could be time to mull whether Federer, who has slipped to No. 7 in the world at 32, and Rafael Nadal, 27, ever will tangle in America's Grand Slam event.

They have dueled 10 times in the three other majors -- eight times in finals and twice in semifinals -- with Nadal ahead 8-2.

At this stage, it isn't clear whether a Federer win over Nadal in a major might inoculate him against the talk about impending retirement. Or if a Nadal win would cement the argument that he -- still behind Federer in major singles titles but with a decisive 21-10 head-to-head lead -- has gotten an edge in the sport's "greatest ever" argument.

Federer has won a record 17 Slam events, Nadal 12, and if Nadal "gets around 15 or 16, then that's a good argument," said Mike Bryan, pursuing a 2013 Grand Slam sweep in doubles with his twin, Bob.

But Monday night, with a Federer-Nadal contretemps a tantalizing round away, Robredo, the 31-year-old Spaniard ranked 22nd in the world, changed the subject entirely. His steadier play and Federer's maddening inability to convert a break-point opportunity -- he got only 2 of 16 -- kept Federer constantly off balance. Federer made 43 unforced errors against Robredo, who was 0-10 against him and had won three of the 27 previous sets they'd played.

Because of the 4 1/2-hour rain delay, they were moved to the second show court, Louis Armstrong Stadium, while Nadal was dispensing with German Philipp Kohlschreiber at Arthur Ashe Stadium, 6-7 (4), 6-4, 6-3, 6-1.

If Federer had held up his end, "It would have been a quarters, not a final" against Nadal, he said. "And if I'm playing like this, I'm not going to beat Rafa. Or Kohlschreiber, for that matter."

He said that neither the extended wait to play nor the venue was an issue. More so that "I didn't show the game that the crowd could really get into and excited about," he said. "Without taking any credit away from Tommy, I kind of self-destructed, which is very disappointing."

Federer did not reach a Slam final this year, the first time that has happened since 2002.

Still true, then: The next Open meeting of Federer and Nadal, five times the tournament's top two seeds, will be the first.

"Really?" Serena Williams said. "Wow. Gosh, that's weird."

Federer won his first Open title in 2004. Nadal's first Slam victory was the 2005 French and, from 2005 through last year, neither was seeded lower than third at the Open (though Nadal withdrew last year).

But while Federer took Open titles in 2005, '06 and '07, Nadal was an early upset victim.

When Nadal finally reached the Open final in 2010 and 2011 (a victory, then a loss, against Novak Djokovic), Federer had been knocked out by Djokovic in the semis both years.

And again, they have avoided the inevitable.

Bryans advance. Top-seeded Bob and Mike Bryan reached the doubles semifinals with a 7-6 (7), 6-4 win over 12th-seeded Colin Fleming and Jonathan Marray.