NEW YORK — If any player needed a break at the U.S. Open, it was Rafael Nadal.

His stomach muscles clearly hurting, the third-seeded Nadal got some time to heal when his match against No. 11 Fernando Gonzalez was postponed because of rain late Thursday night.

Nadal won the first set and was leading 3-2 in the second-set tiebreaker when the second rain delay of the night hit.

Tournament officials waited about 90 minutes before calling play — the first postponement after 10 straight days of fantastic weather at Flushing Meadows.

The postponement, combined steady rain that delayed the start of play Friday, threatened to place the U.S. Open schedule in flux and made it almost certain that the winner of Nadal-Gonzalez would have to pay for the break he got, with a packed schedule through the weekend.

Nadal, the six-time major winner trying to complete a career Grand Slam, has been reluctant to talk about the injury. He doesn't want to let his opponents know his weaknesses. But the medical timeout he took early in the second set, with the trainer poking and massaging his stomach, told the story.

As did his uncle and coach, Toni Nadal.

"In the first set, it was very, very bad," he said in an interview on ESPN2, which is televising the tournament in the United States. "Now, it's a little better. It's difficult. We must win this set."

The delay meant the biggest — and only — winner from Thursday's singles was Juan Martin del Potro. The sixth-seeded Argentine made his second Grand Slam semifinal by beating No. 16 Marin Cilic 4-6, 6-3, 6-2, 6-1 earlier in the day, when the wind was blowing hard in front of the rainstorm that came later.

"I cannot start the match like today," Del Potro said. "I was thinking about other things, and the weather was bad. But it was bad for both players. I just need to be in focus in the beginning of the match until the last point and play my game."

Del Potro and Cilic are quite similar: Both 6-foot-6, with big serves and seemingly unlimited potential. And they were born five days apart in 1988, making them the two youngest players ranked in the top 20.

Cilic raced through the first set and was up 3-1 in the second, when things changed quickly. Del Potro steadied himself in the wind, which reached gusts of 20 mph, and won 17 of the last 20 games.

His first Grand Slam semifinal was at this year's French Open, where he led Roger Federer 2-1 in sets but let it slip away.

"Well, I learn many things. I was so close to beat him," del Potro said. "I have everything to learn, you know. I want to be a good player in the future."

He finished with only 24 unforced errors — 29 fewer than Cilic.

"He was not missing," said Cilic, who upset No. 2 Andy Murray on Tuesday.

The other men's semifinal is already set: No. 1 Federer against No. 4 Novak Djokovic. Federer beat Djokovic at the U.S. Open in the 2007 final and 2008 semifinals, part of an overall 8-4 lead in the head-to-head series.

But the only time Federer has missed a final in the past 17 Grand Slam tournaments was after a loss to Djokovic, who won their semifinal en route to the 2008 Australian Open title.

The men's semifinals are scheduled for what used to be known as "Super Saturday," at the U.S. Open — two semis followed by the women's final.

That schedule, though, depends on the weather.

The women's semifinals were on the slate for Friday. Defending champion Serena Williams was scheduled to meet 2005 champion Kim Clijsters, and Caroline Wozniacki was supposed to face Yanina Wickmayer in a matchup of 19-year-olds both playing in a Grand Slam semifinal for the first time.

Williams is seeking her fourth U.S. Open title and 12th Grand Slam singles championship overall. She also has a chance to win her 10th women's doubles title at a major with older sister Venus. They advanced to that final Thursday with a three-set victory over Alisa Kleybanova and Ekaterina Makarova.