Emma Raducanu retired with a hip injury in the opening round of the Guadalajara Open in her first competitive match since the Australian Open.
The match was already three hours 36 minutes old, making it the longest of the WTA season so far.
Australia's Daria Saville was leading the US Open champion 5-7 7-6 (7-4) 4-3 when the Briton ended the match shortly after taking a medical timeout.
Raducanu has won just one of her three matches this season.
Neither player was at their best in Mexico, with Raducanu saving 16 of 21 break points and Saville 13 of 19.