No. 1 Barty upset in Australian quarterfinals by Muchova

Ash Barty out of Australian Open after capitulating to Karolina Muchova in semi-final

Muchova's only other victory over a top-five was against No. 3 Karolina Pliskova at Wimbledon in 2019.

Karolina Muchova has just explained what happened with her decisive medical timeout early in the second set in her boilover win over Ash Barty in the Australian Open quarter-finals, saying it was taken due to dizziness.

The match may have started in the morning under the beating sun, but the quarter-final delivered enough twists and turns worthy of a primetime match at Rod Laver Arena.

Muchová will now take on either United States young gun Jessica Pegula or Jennifer Brady who will face off later on Wednesday.

"I started feeling a bit lost", Muchova said.

"I tried to get back, played a bit faster rally so we don't play that long ones [like] in the first set and it worked well".

Barty will be left to rue missing out on another golden opportunity to win her home Slam, just a year after her eerily similar semi-final loss to eventual champion Sofia Kenin.

"She played nearly like no mistakes", said Muchova, whose best previous Grand Slam run was to the last eight at Wimbledon in 2019.

The Australian opened up an early break in the second set to lead 2-0 before Muchova took an extended injury timeout and left the court.

"They checked my pressure, they cooled me down and then I went back on the court".

Muchova had not dropped a set in the tournament, but that changed against Barty, who worked break points on the Czech's opening serve and converted the second with a forehand victor.

After coughing up her first double-fault in her next service game, Barty had to fight off another two break points to level the set at 3-3 with Muchova's time-out clearly halting the Australian's momentum.

After dominating the first set hitting six winners to Muchova's one, the world No. 1 hit 19 unforced errors in the second set to her opponent's nine, and then a further 12 in the third to seal her exit. Barty finished with 37 unforced errors and lost serve four times in the final two sets.