WINSTON-SALEM, N.C. (AP) Top-seeded John Isner has withdrawn from his Winston-Salem Open quarterfinal against Lukas Rosol on Thursday because of a sprained ankle.

Organizers say Isner injured his left ankle Wednesday night during his third-round victory over Mikhail Kukushkin of Kazakhstan. Isner won this tournament in 2011 and 2012.

Rosol, a Czech who is seeded seventh, will play ninth-seeded Yen-Hsun Lu of Taiwan or 14th-seeded Andreas Seppi of Italy in Friday's semifinals at Wake Forest Tennis Center.

In other quarterfinal pairings in this tuneup for next week's U.S. Open, Belgian qualifier David Goffin plays Poland's Jerzy Janowicz, and fifth-seeded Guillermo Garcia-Lopez of Spain faces American Sam Querrey.

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