Top seed Kei Nishikori withdrew from the Japan Open Wednesday due to a buttock muscle injury, disappointing home fans hoping he could recapture the trophy. The world number five became the latest star to pull out of the injury-plagued tournament, after US Open winner and defending champion Stan Wawrinka withdrew due to a back injury. Nishikori retired during the first set of his second-round match against João Sousa of Portugal, ranked 34th in the world.