Elina Svitolina retired in tears from her Australian Open fourth-round match against unseeded Czech player Linda Noskova after just three games on Monday.
Svitolina, the 19th seed in Melbourne, needed a medical timeout at 2-0 down in the first set, during which she received treatment on court for her lower back.
The Ukrainian, 29, attempted to continue on Margaret Court Arena but was not moving freely and appeared to be in significant discomfort as her serve was broken for a second time.
The former world number three decided she could not carry on and walked to the net to concede.
Noskova, ranked 50 in the world, is enjoying a breakout tournament, including a stunning win against world number one Iga Swiatek in the third round.
"I feel sorry for Elina and hope she gets well soon," said the 19-year-old, who will play Ukrainian qualifier Dayana Yastremska in the last eight.
