|Kei Nishikori of Japan returns a shot against Joao Sousa of Portugal during the men's singles second round match at the ATP Japan Open tournament in Tokyo on October 5, 2016|
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.
Going into the match, Nishikori looked to be in solid shape, comfortably building a 3-0 lead as he made his rival run back and forth across the hard court at Ariake Colosseum.
But the 26-year-old requested a medical timeout, asking his trainer to massage the left side of his lower back.
Resuming the match, Nishikori was clearly a different man, grimacing at times, although he managed to hold his service game to make it 4-0 before his legs gave up.
Sousa quickly made it 4-3, and Nishikori was forced to withdraw 41 minutes into the match.
The Japanese star, title winner in 2012 and 2014, voiced disbelief at the outcome.
"Until a second ago, I was feeling really good and made it 3-0" before things deteriorated, he told a press conference after the match.
"I still cannot believe what happened."
Sousa will now play against Belgian fifth-seed David Goffin, who defeated Jirí Veselý of the Czech Republic 6-3, 7-5.
Fourth-seed Marin Cilic also defeated Spaniard Fernando Verdasco 4-6, 7-5, 7-5 in the second round.
Gilles Muller of Luxembourg advanced to the second round by defeating Czech third-seed Tomas Berdych, winning the first set in a tie break before dominating the second set to claim victory 7-6 (9/7), 6-1.
Berdych also asked for a medical timeout in the second set, but stayed on to finish the match.