Jack Draper retired from his Japan Open quarter-final against Ugo Humbert after appearing to struggle with an abdominal injury.
The US Open semi-finalist trailed 7-5 2-1 to the Frenchman and was serving when he pulled up during a point.
Draper shook his head and his serve in particular appeared to be hampered by the injury, as he shook hands with Humbert mid-game.
Draper was bidding to reach the semi-finals in Tokyo after securing an impressive win over second seed Hubert Hurkacz in the previous round.
The 22-year-old reached the last four of the US Open last month but struggled with his nerves in the semi-final against Jannik Sinner in New York.
Humbert will take on Tomas Machac for a place in the final, with Arthur Fils, Ben Shelton, Kei Nishikori and Holger Rune also in the draw.
Draper will hope to be fit for the ATP Masters tournament in Shanghai next week, in what is one of the biggest events of the indoor season.
