Britain’s Dan Evans was forced to retire against Frances Tiafoe in Vienna after suffering what appeared to be a serious calf injury on Tuesday.
Evans, who was looking comfortable when leading 4-1 in the first set, pulled up after suffering a sharp pain in the back of his leg.
Despite seeking medical attention, the world No 35 was comforted by American Tiafoe as he pulled out of the contest.
It was only the second tour-level retirement of Evans’s career and it means he will be a doubt for Great Britain’s Davis Cup quarter-final against Serbia next month.
Evans had been expected to partner world doubles No 4 Neal Skupski in Malaga, but his injury could now put pay to captain Leon Smith’s first-choice line-up.
Heather Watson fell in the first round of the Abierto Tampico event in Mexico, losing in three sets to Ann Li.
Watson won the first set 6-4, but the American – ranked 50 places below Watson at 156 – raced through the second 6-1 and took the decider 6-3.
In other news, world No 8 Karolina Muchova has withdrawn from the WTA Finals in Cancun due to a right wrist injury and will be replaced by first alternate Maria Sakkari of Greece.
Czech Muchova was the eighth player to qualify for the season-ending tournament after reaching the Cincinnati final and the US Open semi-finals, losing to American Coco Gauff on both occasions.
“Together with the team and doctors we tried everything till the last moment to resolve my wrist injury,” Muchova said in a statement.
“Unfortunately, the time needed for the recovery is longer than we would’ve hoped for and therefore I had to make this unpleasant decision.”
As a result, the ninth-ranked Sakkari will feature in her third straight WTA Finals at this year’s event, which starts on Sunday, after reaching the semi-finals a year ago.