Andy Murray had to concede victory to Xavier Malisse after suffering a hamstring injury in their third-round match in the Ethias Trophy in Belgium.
The 18-year-old Scot had lost the first set 7-5 to the second seed and home favourite before retiring in order not to do further damage to the problem.
Murray, who has surged up the rankings, has been plagued by fitness problems during his career.
Last week he was runner-up to Roger Federer at the Thailand Open.
"I felt my left hamstring pull and after that I couldn't really move to my backhand well," he said.
"When the ball was high and when it was low I couldn't put any pressure on my leg.
"I got kind of ratty because he started to make a lot of mistakes. At one point I thought, 'if he keeps playing like this, I've got a chance' but then he started to play better and making me run.
"I just thought it was better to stop than to risk hurting myself even more."
Murray's exploits in Bangkok saw him rise to 72 in the world and he was chasing more ranking points at the second-tier Challenger event in Belgium.
He beat Ivo Heuberger 6-4 6-4 in the first round and Gregory Carraz 7-5 6-3 to set up the meeting with Malisse.