Czech 25th seed Karolina Muchova recovers from dizziness to knock out world number one Ash Barty in three sets 1-6, 6-3, 6-2 on Rod Laver Arena.
Neither player had dropped a set going into the contest, but Barty looked to have turned it into a one-sided affair against the Czech player who looked flat and short of confidence.
But it looked to have been illness-related. Muchova took an extended medical timeout early in the second set and returned rejuvenated to shock one of the favourites to win the slam.
Muchova will face either Jennifer Brady or Jessica Pegula in the semis.
The first set was a comfortable affair for 2020 Australian Open semi-finalist Barty who was chasing her maiden Australian Grand Slam title. Barty raced into a 5-0 lead. Muchova saved being bageled by pulling a game back, but the 24-year-old Queenslander wrapped it up in 24 minutes.
The second set started just as quickly with Barty going a break up following a wild Muchova forehand that was her 17th unforced error.
After game three at 2-1, Muchova went off for a medical timeout having appeared unwell. She returned from the locker room after ten minutes entirely refreshed and immediately broke back.
Muchova started playing with far more aggression, coming to the net a lot more, and it scrambled Barty's gameplan. The Australian double faulted at 3-4 down to hand Muchova the vital break and ultimately the set.
Momentum was entirely with Muchova and she broke in the opening game of the third with a stunning passing shot.
The 24-year-old never looked back after that, double breaking at 4-2 up before sealing a place in her first ever major semi-final.
"I was feeling a bit lost by the end of the first set," Muchova said on-court post-match. "Ash started very well she played almost like no mistakes it was very tough.
"I was also a bit lost on the court and my head was spinning so I took a break.
"It helped me and I tried to get back to playing a bit faster so we don't play long rallies like in the first set and it worked well.
"They just checked my pressure because I was a bit lost I was spinning. They cooled me down a bit with ice and it helped me.
"I just tried to put a ball in and go for it. I played a bit faster going to the net. I think that was the key by the end.
"It was the first time [serving for a major semi-final] but I tried not to think about it. To think about just another ball and focus on that. I'm happy that I handled it well."
