Wimbledon semi-finalist Grigor Dimitrov has withdrawn from this week's ATP Washington Open after doctors diagnosed him with the flu and a sinus infection.
Dimitrov had been seeded third for the hardcourt event in the capital behind world No 5 Tomas Berdych of the Czech Republic and sixth-ranked Canadian Milos Raonic.
Frenchman Gael Monfils is another leading player to drop out of the tournament because of an ongoing knee problem.
"We will miss having Grigor as a part of this year's tournament, but his health is priority number one," said tournament director Jeff Newman.
"We hope he gets better quickly and wish him great success for the rest of the year."
The ninth-ranked Bulgarian, who also reached the last eight at this year's Australian Open, has won three of his four career ATP titles in 2014.
The 23-year-old Bulgarian took titles at Acapulco on hardcourt, Bucharest on clay and Queen's on grass. His first tour crown came last year at Stockholm.