Emma Raducanu was forced to retire from her first-round match at the Guadalajara Open after a gruelling battle with Daria Saville.
The US Open champion, who also struggled with her conditioning in her second-round Australian Open match as blisters contributed to her exit in Melbourne last month, took a medical time-out in Mexico before withdrawing in the final set.
At three hours and 36 minutes, Raducanu's match against Saville is the longest of the year so far on the WTA Tour, and both players were clearly physically affected by the time they had spent on court.
Australia's Saville suffered from leg cramps in the latter stages of the meeting, while Raducanu emerged from her time-out with strapping on her left leg.
Raducanu won a marathon 80-minute first set and led 5-3 in the second, only to be broken when serving for the match as Saville levelled proceedings at one-set-all.
The 19-year-old from Bromley was down a break at 3-4 in the final set when she withdrew.
Afterwards, the victorious Saville said: "I worked hard and believed in myself, so I think I won it. This is probably the first match in a long time when I felt like I was being myself. The most important thing was to compete."
Raducanu has only played 11 tour-level events since making her professional bow last year but has suffered a string of physical problems that have contributed to her exiting four tournaments.
Breathing difficulties forced Raducanu out of her Wimbledon fourth-round meeting with Ajla Tomljanovic, before she was soundly beaten by Marta Kostyuk in the quarter-finals of the Transylvania Open due to illness.
Severe blistering on her right hand was a contributing factor in her Australian Open second-round defeat by Danka Kovinic in January.
