With an injured rib smarting, Kei Nishikori kept going until he couldn't stand it any longer.

The time to call it quits came Monday in the fourth round at Wimbledon, trailing 6-1, 5-1 against Marin Cilic on No. 2 Court.

"Even before the match, I knew it's not going to be easy for me to play 100 percent," Niskikori said. "Every point I played, get worse. I didn't want to, but I guess I had to retire today."

The fifth-seeded Japanese player warmed up for Wimbledon by playing on grass in Halle, Germany, but he pulled out before his second match because of the rib injury.

He said it was that same injury that forced him out of Wimbledon.

"It's been not recovering well these two weeks," Nishikori said. "Today, I was hoping to get better, but it was too much pain on the court."

Nishikori has never reached the quarterfinals at the All England Club, but he did play in the 2014 U.S. Open final, losing to Cilic. The ninth-seeded Cilic will next face Roger Federer at Wimbledon.

Nishikori said he is hoping to be healthy again in time for the buildup to the final Grand Slam tournament in New York.

"Next tournament is Toronto. I'm going to have three weeks," Nishikori said. "Hoping to get better soon."