Kei Nishikori says he has been having breathing problems, affecting his performance on court.
The No. 5 fell to fellow Japanese Yoshihito Nishioka in the second round at Cincinnati, his third straight defeat.
"I've been having trouble breathing since Wimbledon, so I've got to get it checked," he said.
Nishikori also fell in his opener at Montreal, going more than three hours against Richard Gasquet. Before that, he had reached the quarterfinals at Wimbledon, falling to Roger Federer.
Nishikori was the highest seed to exit in the second round at Cincinnati.