Elina Svitolina exited the Australian Open in tears after just three games on Monday.
Svitolina, who was playing her fourth-round match against unseeded Czech player Linda Noskova, retired due to a lower back injury.
Svitolina, the 19th seed in Melbourne, took a medical timeout at 2-0 down in the first set.
The Ukrainian, 29, tried to continue playing but was not moving freely and appeared to be in pain as her serve was broken for a second time.
Soon after, the former world number three walked to the net to concede the match.
Noskova, ranked 50 in the world, is having a great 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.
Yastremska, who beat Wimbledon champion Marketa Vondrousova in the first round, defeated another former Grand Slam champion with a 7-6(6) 6-4 victory over 18th seed Victoria Azarenka.
On the back of a solid backhand, the world number 93 came from behind in both sets and smacked 38 winners on Rod Laver Arena.
"I thought that I had lost this match 25 times," the 23-year-old said after her victory over the Belarusian.
"I played pretty aggressive, I think. In some moments I felt like I was too nervous and too emotional, but then I just relaxed and said, 'it's going to be like it's going to be'."
Yastremska, whose grandmother's house was hit by a rocket while she was playing a warm-up event in Brisbane, said the Russian invasion of Ukraine had made life difficult for her in the past couple of years.
"It's tough emotionally to play, but the worst thing is you feel like you were already accepting this, what is happening," she added.
"And people are starting to forget about what is going on ..."
