British number one Emma Raducanu was hampered by more physical problems as she missed out on a place in the Madrid Open quarter-finals with defeat by Ukraine's Anhelina Kalinina.
Raducanu, aiming for three straight wins for the first time since her 2021 US Open triumph, lost 6-2 2-6 6-4.
The 19-year-old was "struggling" with a back issue and needed treatment at the end of the first set.
"I would have given myself a 5% chance of winning that match," she said.
"For it to almost happen, I think that was a positive thing."
Raducanu, ranked 11th in the world, showed determination to fight back but Kalinina regrouped to seal victory.
Raducanu saved a match point as Kalinina tightened up with victory in her sight, but hit long to give the Ukrainian another chance and then volleyed wide.
But the back issue is the latest physical problem to affect Raducanu as she navigates her first full year on the WTA Tour.
She struggled with a blister on her racquet hand in a Australian Open second-round defeat in January and also retired with a leg injury from a lengthy opening-round match in Mexico in February.
The British teenager's Billie Jean King Cup debut ended in painful fashion last month when she was hampered by a blister on her foot - which required bathing her feet in surgical spirit every day.
"Throughout the week I have been carrying some niggles, and it's kind of just taking its toll, all of the matches at this kind of level," said Raducanu, who is optimistic of playing in Rome next week.
"I think that's a good thing that I feel like I'm going through this and my body is building with each match I play."
Despite the defeat, Raducanu smiled as she walked off court and will take positives from her time in the Spanish capital.
After only making her senior clay-court debut last month, the world number 11 is continuing to look comfortable on a surface which she believes she can thrive on.
The teenager, who is without a full-time coach after parting with Torben Beltz recently, also showed her ability to problem solve as she fought back against Kalinina in the second set.
Kalinina, ranked 37th after a rapid rise in the past year, broke to love in the first game of the match and continued to dominate with her powerful ball-striking, particularly causing Raducanu problems with her backhand.
When she returned to court after treatment, Raducanu tweaked her strategy to combat Kalinina's heavy returning - and it paid dividends.
She mixed up her serving and landed 78% of her first serves, as well as making just one unforced error, to dominate the second set and force a decider.
Momentum swung again when Kalinina broke for a 2-1 lead, with further twists coming when Raducanu levelled at 3-3 and Kalinina broke again for 5-4 with a sensational crosscourt backhand winner.
That proved pivotal as Kalinina, who has now beaten three Grand Slam champions in Madrid after previous victories against Sloane Stephens and Garbine Muguruza, served out to set up a quarter-final against Switzerland's Jil Teichmann.