Ons Jabeur suffers injury scare days before Australian Open
Jabeur was playing the in-form world number seven Anett Kontaveit, who won a tight first set 6-4 before the Tunisian called for a medical timeout.
After lengthy treatment on her lower back, Jabeur forfeited the match.
It is a big setback for Jabeur, with the opening Grand Slam of the year starting on Monday. She had a breakthrough 2021, when she won her maiden WTA singles title and surged into the top 10 for the first time.
Kontaveit will now play world number four and French Open champion Barbora Krejcikova, with the Czech crushing France's Caroline Garcia 6-0, 6-2.
"It's never the way you want to win and I really hope Ons is going to be fine for next week and that it's nothing too serious," said the Estonian.
"The first set was really competitive and we've had some tough matches before and she's always come out on top, so it was good to get the set and that I was playing the way I was playing."
The understated Kontaveit is shaping up as a serious contender at Melbourne Park after her electric form in the second half of last season, capturing four titles in two months before ending runner-up at the WTA Finals to Garbine Muguruza.
Krejcikova also raced through the rankings last year, winning at Roland Garros, Strasbourg and Prague.
"I'm really looking forward to the challenge to play another top-10 player," she said of Kontaveit. "I'm glad to be putting myself in these situations and getting good matches and getting quality time on court."
Spanish world number three Muguruza faces Russia's Daria Kasatkina later Thursday for a place in the semi-finals.
The last four line-up will be completed when fellow Spaniard and fifth seed Paula Badosa meets Tokyo Olympics gold medallist Belinda Bencic.
Among the men, Russia's Aslan Karatsev defeated Italian Lorenzo Sonego 6-2, 3-6, 6-2 to make the last four while British veteran Andy Murray enjoyed an easier passage.
The former world number one, who has been given a wildcard into the Australian Open, was back in the changing rooms after one set when David Goffin retired after losing the opener 6-2.
Murray will face Reilly Opelka for a place in the final after he beat fellow American Brandon Nakashina in straight sets.