Top seed and home favourite Kei Nishikori was forced to retire from his second-round Japan Open match against Portugal's Joao Sousa on Wednesday after he sustained a buttock muscle injury while leading the first set 4-3.
Having won the tournament in 2012 and 2014, the 26-year-old Nishikori was chasing his third Japan Open title, which would have put him one behind Stefan Edberg's record four wins in Tokyo.
"It was hugely disappointing to have to retire from my match," the world number five said.
"It is possible that the injury was due to an accumulation of fatigue and stress, but I came into this tournament with a lot of rest and feeling very ready. I will get examined and hope to recover in time for my next tournament."
Nishikori is next due to play in the Shanghai Masters starting on Oct. 9.
(Reporting by Shravanth Vijayakumar in Bengaluru, editing by Ed Osmond)