"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.
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(Reporting by Shravanth Vijayakumar in Bengaluru, editing by Ed Osmond)
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