BERLIN (AFP) - Japan's world No. 6 Kei Nishikori suffered a pre-Wimbledon blow when he was forced to pull out of the ATP Halle tennis tournament on Wednesday with a rib injury.

Nishikori, seeded second in Germany behind Roger Federer and a semi-finalist in 2014 and 2015, was to play Florian Mayer.

"I'm very sad to not be able to go on playing," he said.

It was not immediately clear if he would be fit for Wimbledon, which begins in under two weeks.