Australia's world No.1 capitulated from a set and a service break up in a shock 1-6 6-3 6-2 quarter-final loss to Czech Karolina Muchova on Wednesday.

Barty never recovered after losing nine out of 11 games mid-match after the 27th-ranked Muchova took a tactical medical time-out while trailing the top seed 6-1 2-1 on Rod Laver Laver Arena.

"I was a bit lost on the court and my head was spinning so I took a break and it helped me," Muchova said.

"I started feeling a bit lost by the end of the first set. Ash started very good. She played like no mistakes. It was very tough."

There was no sign of trouble for Barty early on as the top seed dropped just seven points while charging to a 5-0 lead in the first set.

Muchova took 21 minutes to finally get on the board.

Not letting up, Barty broke Muchova for a third time in the opening game of the second set to continue her utter domination.

Then the quarter-final turned spectacularly.

Muchova called for the tournament doctor after holding serve to trail 2-1, then took her medical time-out after having her pulse and temperature checked.

Asked if she was injured, the 2019 Wimbledon quarter-finalist said: "No. It was more they just checked my pressure because, like I said, I was a bit lost. I was spinning.

"So they cooled me down a bit with ice and it helped me."

The world No.27 returned to the court several minutes later and swiftly broke Barty's serve for the first time.

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.

Mixing deep moon balls with delicate backhand drop shots, the Czech grabbed another service break to surge to a 5-3 lead before snaring the set to level the match up.

Barely missing a ball as Barty's unforced error tally mounted, Muchova broke the Queenslander twice more in the deciding set before converting her first match point with an ace after an hour and 57 minutes.

Barty's departure extends the local singles title drought to a 44th year.

Muchova will return on Thursday to play the winner of the all-American quarter-final between Jennifer Brady and Jessica Pegula for a place in Saturday's title match.

Australian Associated Press