Two-time champion John Isner withdrew from the US Open tune-up event at Winston-Salem on Thursday with a left ankle injury. The American was due to play Lukas Rosol in the quarter-finals on Thursday, but pulled out with the injury suffered in his win over Mikhail Kukushkin on Wednesday. "I am extremely disappointed to have to withdraw from the Winston-Salem Open, one of my favourite stops of the year," said Isner, who hails from North Carolina.