(Reuters) - Japan's Kei Nishikori pulled out of the Gerry Weber Open with a rib injury on Wednesday, tournament organizers said.

The world number six, and second seed for the grass court tournament, had been due to face German's Florian Mayer but was not fit enough to take to court in Halle.

"I'm very sad to not be able to go on playing," Nishikori, who needed treatment during his first-round win against Lucas Pouille, told the tournament's official website.

Nishikori said he was confident the injury would improve before the start of Wimbledon on June 27.

(Reporting by Martyn Herman, editing by Pritha Sarkar)