Kei Nishikori has once again left the Gerry Weber Open with fitness questions.
"It's really too bad," Nishikori said. "I'm surprised because there was no hint (of injury).
The 27-year-oldretired from his second-round match against Russia's Karen Khachanov while trailing 3-2 in the first set on Thursday. Nishikori began walking gingerly in the fourth game after he brought up match point against the 38th-ranked Khachanov.
"It felt like I twisted it," said Nishikori, who called for a medical timeout at the side change after Khachanov held serve and received a massage to his left hip and lower back.
But the world No. 9 gave up before completing his ensuing service game. According to a doctor, the pain in Nishikori's glute muscles will likely require a week of treatment.
The tournament was Nishikori's first grass-court event since his clay-court season ended at this month's French Open.
Wimbledon starts on July 3.
In 2015, Nishikori was knocked out here by an injury to his left leg, while last year it was an injury to his left oblique muscles.
"The only times I have excess movement is when I'm in good form," Nishikori said. "You get irregular bounces on grass, and your body moves suddenly. I think that's a strong possibility behind this (injury issue)."