Tobias Kamke's Madrid Masters ended in disappointment with the German forced to retire in his round-of-32 clash with Nicolas Almagro on Monday.
There were no signs of injury for Kamke in the first set against his Spanish opponent, with just one break of serve allowing Almagro to move into a one-set lead.
Kamke, who received a period of treatment at the end of the first, did begin the second, but was forced to retire after just five minutes of the second set.
It is not yet clear what type of injury troubled the German, who is currently ranked world number 82.
