The only Japanese player ever to be ranked in the world’s top five, Nishikori was due to face Argentine Leonardo Mayer in the second round on Sunday. He has been replaced by lucky loser Ruben Bemelmans of Belgium.
“I’m still not strong enough to go play a match,” Nishikori said. “I’m very sorry as I love this tournament.”
The 28-year-old had returned to play this winter after a five-month injury layoff due to a wrist injury.
Nishikori became the first men’s player representing an Asian county to reach a grand slam singles final at the U.S. Open in 2014 but lost in straight sets to Croat Marin Cilic.
Reporting by Gene Cherry in Salvo, North Carolina; editing by Ken Ferris