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 favorite stops of the year," said Isner, who hails from North Carolina.
It wasn't yet clear if the injury would affect 29-year-old Isner's appearance in the US Open, which starts on Monday in New York.
Isner is seeded 13th at the Open, and according to the draw released on Thursday could face top seed Novak Djokovic in the fourth round.
Rosol advanced by walkover into a Friday semi-final against Taiwan's Lu Yen-Hsun, who defeated Italy's Andreas Seppi 6-4, 6-4.
The other semi-final will match Poland's Jerzy Janowicz, who ousted Belgian David Goffin 6-4, 6-2, against American Sam Querrey, who rallied to eliminate Spanish fifth seed Guillermo Garcia-Lopez6-7 (3/7), 6-2, 6-4.