Czech Jiri Lehecka (22) progressed to the semi-finals of the Madrid Open on Thursday after world number four Daniil Medvedev (28) quit the match with a groin problem after losing the first set.
Third seed Medvedev requested a medical time out due while leading 3-2 and continued to visibly struggle to move after coming back onto the court.
Lehecka, who knocked out Rafa Nadal in the last 16, then broke Medvedev to take a 4-5 lead and took the set 6-4 before Medvedev told the umpire he could not continue the match.
Lehecka will face Canada's Felix Auger-Aliassime in the semi-finals on Friday.
The 23-year-old Canadian was due to face top seed Jannik Sinner in the quarter-finals but the Italian pulled out from the tournament with a right hip injury on Wednesday.
Russian seventh seed Andrey Rublev will play Taylor Fritz, seeded 12th, in the other semi-final.