Emma Raducanu struggled with her conditioning as she lost her third-round match at Indian Wells while competing with a stiff back.
The reigning US Open champion won the first set against Petra Martic on a tie-break but went on to lose 6-7 (3-7) 6-4 7-5.
Raducanu, 19, has won just four matches in the last six months, and fitness issues have blighted the early stages of her career.
Breathing difficulties forced Raducanu out of her Wimbledon fourth-round meeting with Ajla Tomljanovic last year, before she was soundly beaten by Marta Kostyuk in the quarter-finals of the Transylvania Open due to illness.
Severe blistering on Raducanu's right hand was a contributing factor in her Australian Open second-round defeat by Danka Kovinic in January, and she retired from her first-round match at the Guadalajara Open last month.
Speaking after the loss to Martic, Raducanu said: "I think it's just part of not playing and having so much stop-start and having five, six days to prepare for matches and then playing at that intensity.
"It was a tough one not being able to serve full out so I was having to work so hard just to hold. I've just got to get stronger.
"I've had it (stiff back) for the last few days, just a product of training hard and probably the last match was pretty intense too and it didn't settle down.
"When you are crossing your fingers to hold serve every time, it's tough. I couldn't really drive up and I was kind of struggling to turn and reach. The margins were so small."
Raducanu said she expects to be fit for the Miami Open, which gets underway next week.
In better news for British tennis, qualifier Harriet Dart, 25, reached the last 16 with a win over Kaia Kanepi.
Dart, who overcame 12th seed Elina Svitolina earlier in the tournament, will move into the top 100 for the first time when the women's rankings are updated next week.
"My ranking might not show where my level is, but I've always believed my level has been there," Dart said.
"I put in the work day in, day out, and finally to be able to put a run like this together, it's not by chance.
"I always have nothing to lose - especially at these events. It's kind of a bonus to be able to play another match in this amazing place. It's quite surreal. I'm enjoying every minute of it."
