WINSTON-SALEM, N.C. — John Isner’s preparations for the U.S. Open hit a setback Thursday when a sprained ankle forced him to withdraw from the Winston-Salem Open hours before his quarterfinal.

Isner, seeded 13th for next week’s tournament at Flushing Meadows, was to have played Lukas Rosol of the Czech Republic.

At No. 15, Isner is the highest-ranked American in the U.S. Open. He is to face Marcos Giron, the 2014 NCAA singles champion, in the first round.

“I can’t risk further injury with the U.S. Open right around the corner,” Isner said during a news conference at the Wake Forest Tennis Center. “It was pretty uncomfortable doing even minimal stretching on it. I didn’t see the benefit of taking the court.”

Isner’s withdrawal sends Rosol to Friday’s semifinals, where he will play ninth-seeded Yen-Hsun Lu of Taiwan, a 6-4, 6-4 winner over 14th-seeded Andreas Seppi of Italy.

The other semifinal will have American Sam Querrey take on Poland’s Jerzy Janowicz. Querrey advanced by beating fifth-seeded Guillermo Garcia-Lopez of Spain 6-7 (3-7), 6-2, 6-4. Janowicz beat Belgian qualifier David Goffin 6-4, 6-2.

Isner, who won the Winston-Salem title in 2011 and 2012 and was seeded first, injured his left ankle Wednesday night during his third-round victory over Mikhail Kukushkin of Kazakhstan.

“It happened at 5-all,” Isner said. “I went over on it pretty good. But immediately after it happened, I felt no pain on it at all. I was a little worried when I went to bed that night, and it was pretty swollen this morning. There’s not much motion in the ankle right now, which led me to this decision.

“It’s going to impact my preparations. I probably won’t get on the court and hit some balls until Saturday. But I’m going to do everything I can to get it as fit as possible. I certainly expect to take the court in my first-round match, and at this point I expect to play well up there as well.”

Connecticut Open

Kvitova moves into semifinals: In New Haven, Conn., Wimbledon champion Petra Kvitova moved into the semifinals of the Connecticut Open with a 6-4, 6-1 win Thursday over fellow Czech player Barbora Zahlavova Strycova.

Kvitova went down by a service break in the third game of the match, but avoided the upset bug that has sent every other seeded player in the tournament packing.

Trailing 4-3, the second seed called her coach onto the court, and he told her to stop trying to win on every shot and be more patient.

“So I was trying to play a little bit longer rallies, just wait for the good point that I can really attack her, going forward for the final volleys, for example,” she said.

It worked. She broke back to tie it at 4, and then broke Zahlavova Strycova again to take the set.

She cruised in the second, taking the final five games of the match.

Kvitova, who won here in 2012 and lost in the finals last year to Simona Halep, said she’s not treating this tournament as a glorified practice for next week’s U.S. Open Slam— something other big names have been known to do.

“I want to play my best tennis and have the great result I can,” she said. “Every match I step on the court, I want to win.”

Her next match is against Australian Samantha Stosur. The 2011 U.S. Open champion beat Kirsten Flipkens of Belgium 6-3, 4-6, 6-3.

After struggling with Flipken’s serve all match, Stosur broke her to go up 4-3 in the final set on a nice inside-out forehand, and then broke her again in the final game of the match.

“The last couple of games, I just tried to return a little bit smarter with where I was trying to place it,” she said. “Once I was able to hit a return to a better part of the court, then I felt like I was in control of the point more.”

On the other side of the bracket, Magdalena Rybarikova of Slovakia ousted American Alison Riske in three sets, 7-5, 0-6, 6-4. Rybarikova, who beat the top-seeded Halep in the second round, broke Riske’s serve six times. But she failed to hold her own serve seven times, and said she felt lucky to escape with the win.

“I was really fighting, trying to somehow make her play worse, to make her to make more mistakes than me,” she said. “It kind of works.”

Riske, who was the last remaining American in the draw, said she made too many errors in the third set. Three double faults in one game put her down 4-2 and she was unable to climb all the way back.

“Obviously at this level, that can’t happen,” she said. “That was a huge problem.”

Rybarikova will face hard-serving Italian Camila Giorgi in the semis. Giorgi, who upset Caroline Wozniacki in the second round, got by Garbine Muguruza of Spain, 6-4, 6-7 (4), 6-2.

Giorgi, who has climbed to No. 38 in the world rankings, double-faulted 19 times, but compensated with a strong return game, breaking Muguruza nine times.

“My type of game, it’s more secure when my serve goes in, and it’s easier,” she said. “But It’s not a problem. ... I mean, I have the return.”