Ashleigh Barty lunges to play a forehand in her match against Karolina Muchova at the 2021 Australian Open on Wednesday in Melbourne, Australia. Photo: VCG

World No.1 Ashleigh Barty's dream of becoming the first homegrown Australian Open champion since 1978 was shattered on Wednesday after an extraordinary 1-6, 6-3, 6-2 quarterfinal loss to Karolina Muchova.

The Czech was a player transformed after taking a medical timeout when she started feeling dizzy in the Melbourne heat and rallied to secure a place in her first Grand Slam semifinal.

The 24-year-old will meet Jennifer Brady in the last four after the 22nd seed beat her fellow American and close friend Jessica Pegula 4-6, 6-2, 6-1, for a place in the final against Serena Williams or Naomi Osaka.

Barty had no complaints about the medical timeout that turned the match on its head.

"It's within the rules," Barty said. "She's within her rights to take that time. If she wasn't within the rules, the physios and the doctors would have said so.

"I was disappointed that I let that become a turning point. I'm experienced enough now to be able to deal with that."

Barty had started the contest like a woman possessed and conceded only six points in racing to a 5-0 lead in just over a quarter of an hour, wrapping up the first set soon afterwards.

Muchova had some treatment at her chair early in the second set and then took the lengthy medical timeout, leaving the former French Open champion trying to stay loose by doing stretches on court.

"I started feeling a bit lost by the end of the first set," said Muchova.

"My head was spinning so I took a break... It was more they just checked my pressure. As I said, I was a bit lost, I was spinning. They cooled me down a bit and it helped me..."

Although not as dominant as the Australian had been in the opening set, the Czech quickly levelled up the match at 1-1 to become the first player to take a set off the top seed at this tournament.

Barty continued to carve out chances on Muchova's serve but failed to convert them and her error count soared as she tried to finish points too early to wrest back the momentum.

Muchova, the 25th seed, hit 17 winners over the match and settled her nerves sufficiently to serve out for the win at the first time of asking, sealing her place in the last four with an ace.

"I just tried to put a ball in and go for it, play a bit faster, go to the net," Muchova added in explanation of her transformation.

The canned applause piped into the arena in the absence of fans appeared to taunt Barty, who would have been guaranteed passionate support for her semifinal had she made it through after Melbourne's coronavirus lockdown was lifted. "Tough one today, without a doubt," Barty added. "I would have loved to have done a little bit better. Not my day today. We'll go back to work and keep trying to grow for tomorrow."