NEW YORK (AP) – Rafael Nadal felt the pain sear into his right knee early in his U.S. Open semifinal, on what he called “a bad movement.” It was a familiar pain, one that he’s dealt with off-and-on for years.

The defending champion looked up at his guest box and indicated something was wrong. He tried to continue. Eventually, he could not.

Nadal stopped playing after dropping the opening two sets Friday night, putting Juan Martin del Potro back in a Grand Slam final for the first time since winning the 2009 title at Flushing Meadows.

“That was not a tennis match at the end. Just one player playing, the other staying on one side of the court,” Nadal said. “I hate to retire, but staying one more set out there, playing like this, would be too much for me.”

Del Potro was leading 7-6 (3), 6-2 after two hours of play when Nadal shook his head and said he had to stop.

“Of course, it’s not the best way to win a match,” said del Potro, who hugged Nadal when it was over. “I don’t like to see him suffering on court today. So I’m sad for him.”

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