Kei Nishikori has been forced to retire from the Japan Open after suffering a buttocks injury during his second-round match with Joao Sousa.

The 26-year-old - a two-time winner of the tournament in Tokyo - was 4-3 up in the first set when he made the decision to withdraw from the contest.

"It was hugely disappointing to have to retire from my match," Nishikori told reporters at a press conference afterwards.

"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's withdrawal leaves second seed Gael Monfils as the bookies' favourite to win the tournament.