REPORTING FROM NEW YORK — Maybe a matchup between Roger Federer and Rafael Nadal at the U.S. Open just isn't meant to be.
Their splendid rivalry began in 2004 and has played out 37 times around the world but has never included a meeting at Flushing Meadows. That seemed destined to change when they were placed in the same half of the draw this year, Nadal as the No. 1 seed and Federer at No. 3, and both kept the possibility alive by overcoming early stumbles to reach the quarterfinals. Nadal advanced to the semifinals Wednesday afternoon by losing only five games in dismissing Russian teenager Andrey Rublev. Federer, admittedly edgy and unsure he had the physical and mental resources to win, fell short of bringing about that long-awaited matchup here.
No. 24 seed Juan Martin del Potro of Argentina, who had beaten Federer in the 2009 U.S. Open final but has since been cursed with wrist problems and multiple surgeries, upset Federer 7-5, 3-6, 7-6 (8), 6-4 before an enraptured crowd Wednesday night at Arthur Ashe Stadium. Federer, blunt in assessing his shortcomings, said it was the right outcome even if sentiment had leaned toward seeing him play Nadal.
"I didn't even think about it, as I lost that match, that it's not going to happen. I'm dealing with just trying to understand what happened and just to overcome this in the next few hours, days, weeks, whatever it is. I'll be fine," Federer said.
"Of course it is a pity, but Juan Martin deserves it more. I feel I have no place in the semis and he will have a better chance to beat Rafa, to be honest. The way I played or am playing right now, it's not good enough, in my opinion, to win this tournament. It's better I'm out and somebody else gets a chance to do better than me."
Federer said his balky back, which had bothered him earlier this summer, was not a problem Wednesday and had improved over the course of the tournament. But after he needed five sets to defeat Frances Tiafoe in the first round and five to defeat Mikhail Youzhny in the second round he knew he would be unlikely to earn his 20th career Grand Slam title here.
It was also his bad luck that Del Potro had made it out of the fourth round. The big-serving Argentine was sick enough during his match against Dominic Thiem on Monday to consider retiring in the second set but gutted out an emotional five-set win. Del Potro seemed perfectly healthy Wednesday when Federer tried to tire him out early with a barrage of drop shots, and Del Potro's backhand might have been its strongest in years, attesting to the strength of his wrists.
"It was one of those matches where if I ran into a good guy, I was going to lose, I felt. I don't want to say I was in negative mind-set, but I knew going in that I'm not in a safe place," said Federer, whose uncertainty showed in the third-set tiebreaker when he wasted four set points. He lost the tiebreaker on one of Del Potro's many effective forehands and his own errant backhand volley.
"Might have depended too much on my opponent, and I don't like that feeling," Federer added. "I had it, you know, throughout the tournament, and I just felt that way every single match I went into."
Del Potro had the support of a large contingent of Argentine supporters, who roared as he pumped his fists in celebration.
"Tonight I play just free. I don't have nothing to lose against Federer, so I do my best game," Del Potro said. "It was so important to me beating him another time in this amazing tournament."
Nadal said he must be aggressive against Del Potro, whose serve he called the fastest on the tour. "If you let him play from good positions with his forehand you are dead because he plays super-aggressive, hitting so hard," Nadal said.
The winner will face the winner of the semifinal between Pablo Carreno Busta of Spain and Kevin Anderson of South Africa. Del Potro, while respectful of Nadal, isn't conceding anything. "I know if I play my best tennis I could be a danger to him," Del Potro said.