Juan Martin del Potro advances at U.S. Open with victory over Marin Cilic

Roger Federer is an extraordinarily talented tennis champion. And right about now he’s also a very lucky one.

For the first time in this U.S. Open, rain arrived Thursday and it came at a bad time for third-seeded Rafael Nadal and 11th-seeded Fernando Gonzalez.

With Nadal leading 7-6 (4), 6-6 and 3-2 in the tiebreak, a match that had already been stopped once for 1 hour 15 minutes because of rain was finally postponed at midnight until today after another 90-minute delay.

Meanwhile, the top-seeded and five-time defending champion Federer and fourth-seeded and three-time semifinalist Novak Djokovic enjoyed their Wednesday quarterfinal and will have had two full days off before Saturday’s semifinals.

Argentine 20-year-old Juan Martin del Potro also sneaked into the semifinals before the bad weather arrived. On a gloomy, windy afternoon, sixth-seeded del Potro beat 16th-seeded Marin Cilic of Croatia, 4-6, 6-3, 6-2, 6-1. Del Potro will play either Nadal, who is struggling with an abdominal injury, or the 29-year-old Gonzalez.

The weather forecast for today is also not positive, with the chance of rain predicted as 90%.

If Nadal and Gonzalez can’t finish, it would almost certainly mean a Monday final for the second straight year. Federer and Andy Murray played a Monday final in 2008 after rain had forced a stoppage of Nadal’s semifinal against Murray.

Also scheduled today are the women’s semifinals.

Earlier Thursday, Venus and Serena Williams had roused the modest morning crowd by winning their semifinal doubles match, 7-6 (4), 3-6, 6-2, over a Russian team of Alisa Kleybanova and Ekaterina Makarova. It was a spirited workout for defending singles champion Serena, who will play the first women’s semifinal at 9:30 a.m. PDT against unseeded Kim Clijsters.

After Nadal and Gonzalez play the rest of their quarterfinal, the second women’s singles semifinal will be between a pair of 19-year-olds, ninth-seeded Caroline Wozniacki and unseeded Yanina Wickmayer of Belgium.

Williams, 27, has a 7-1 career advantage over the 26-year-old Clijsters but the two haven’t played since 2003, partly because Clijsters just returned to tennis last month after a two-year retirement.

But some things haven’t changed since she has been gone, Clijsters said.

“She’s always been able to step it up at the Grand Slams,” Clijsters said. “I’ve seen her play here and she has that face where she’s like, ‘OK, I’m here to do business.’ ”

Williams is the only semifinalist who hasn’t lost a set during the tournament, and she has already won two of the first three major tournaments this year -- the Australian Open and Wimbledon.

Wickmayer and Wozniacki are both making their first Grand Slam semifinal appearances; the teenagers have never played each other. Wozniacki, the first Danish woman to go this far in a Grand Slam tournament, has won 61 matches this year, most by anyone on the WTA Tour.

There hasn’t been much winning this summer for Nadal. He missed the chance to defend his Wimbledon title because of knee tendinitis and he has been bothered by an abdominal strain he suffered last month in Cincinnati.

Nadal, 23, called for the trainer after he won the first set and pointed to his stomach. He had also needed the trainer last week for treatment to the same area.

Del Potro and Cilic were the two youngest quarterfinalists, and it was Cilic who had already played once on the Ashe court in a fourth-round upset of second-seeded Murray. And that experience of competing in a 23,000-seat stadium seemed to help Cilic early. He was the aggressor.

But Del Potro, who was a French Open semifinalist earlier this year, said, “I was thinking to make every point the same, to put the ball on the court. When you are going to fight until the end you will have many chances.”

Del Potro is 16-1 in matches since Wimbledon. He said there’s a reason. “I like to play here in the U.S., and I like hard courts,” he said. “I like this tournament, I like everything. The stadium, the crowds, the people, the city. Everything. It’s so lovely.”

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diane.pucin@latimes.com