WINSTON-SALEM, N.C. (AP) -- 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.''
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.''