– Rafael Nadal felt the pain sear into his right knee early in his U.S. Open semifinal, on what he called "a bad movement."

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."

The No. 1-seeded Nadal has a history of tendinitis in his knees, and he has often cited that when withdrawing from tournaments. He was visited by a trainer at the changeover after the match's seventh game and tape was applied below the joint.

At the next changeover, though, Nadal pulled off the tape. After the third game of the second set, he had a medical timeout.

But Nadal's movement was clearly limited. At one juncture, he approached the chair umpire to complain about a late call from a line judge and mentioned in passing that he was going to have to quit. Soon enough, he did just that.

Nadal said he didn't know what kind of effects might have been lingering from his quarterfinal victory over Dominic Thiem, which lasted five sets and nearly five hours. But he had a knee issue earlier in the tournament, when he had it taped during his win against Karen Khachanov in the third round.

For del Potro, who will face two-time champion Novak Djokivic — a 6-3, 6-4, 6-2 winner over Kei Nishikori — it was an odd way to return to an important summit. Nine years ago, he stunned Nadal in the semifinals, then Roger Federer in the final, to win the U.S. Open at age 20. Then came a series of wrist operations — one on his dominant right arm, and three on his left — that slowed his career and kept him out of two years' worth of major tournaments.

He has returned to the height of his powers and the height of his sport, up to a career-best No. 3 in the rankings.