Australian world No.1 Ash Barty has been knocked out of the Australian Open after Czech 25th seed Karolina Muchova mounted a remarkable 1-6 6-3 6-2 comeback victory.

Muchova was dishevelled as the 2019 French Open champion raced to a one-set lead in 25 minutes, and Barty had the wind in her sails after breaking for a 2-1 lead in the second.

The Czech, making her second major quarter-final appearance, called the trainers for an assessment before walking off court for a 10-minute medical timeout, having only hit one winner to 18 unforced errors.

That marked the start of an extraordinary resurgence inside a crowd-less Rod Laver Arena (more on that later).

Muchova rampaged through 11 of the last 14 games of the match as she elevated – and Barty played tentatively – to book her place in a maiden grand slam semi-final.

Post-match, Muchova admitted she took the medical timeout as she felt she was “a bit lost”, her “head was spinning” and was treated with ice packs to cool her down.

It was very tough and I was a bit lost on the court and my head was spinning so I took a break. It helped me,” Muchova said.

“I tried to get back, played a bit faster rallies so we don’t play the long ones as in the first set and it worked well.”

The top seed was not drawn on whether the match timeout was in the spirit of the game under repeated questioning from local reporters.

“For me, that’s not really my decision and not my concern what she took the medical for,” Barty said.

“I don’t write the rules. I abide by them. All of us players, we abide by the rules that are written.”

But she said the timeout shouldn’t have been such a crucial element to the match.

“I’m disappointed it did become a turning point,” she said.

Barty, who was bidding to become the first Australian woman to win her home grand slam in 43 years, raced out to the early lead as her heavy forehand ran her Czech opponent ragged.

Karolina Muchova served her way into the final four after a stunning comeback. Photo: Getty

Muchova failed to execute an early strategy of pummelling groundstrokes to the Barty backhand as rivers of errors flowed, and would conclude the set with 13 easy mistakes to her name.

Meanwhile, Barty’s all-court game was cleaner than the sanitised stadium the pair were playing in, notching up a tidy six winners to match her unforced error count.

After racing away with the first set courtesy of a ripping lunging forehand passing shot and some soft hands at the net, she seemed primed to reach her second final four in Melbourne in as many years.

But after the Czech’s timeout – most of which Barty spent rotating her arms and squatting in the Melbourne sun – the script was rewritten with a dramatic plot twist.

Simple errors emerged from the Australian as she never regrouped, and the Czech smelled blood.

Muchova out-Bartied Barty, making daring ventures to the net, swatting away five smashes in succession at one point and gaining the upper hand in sliced rallies against the best backhand cutter in the game.

The first signs of a stunning result materialised when Muchova raced to a 0-30 lead in Barty’s third service game, and although failing to convert a break point with some terrific wheels from the Australian to dig out a return off a well-placed drop shot, seized an opportunity in the next.

That break was decisive as Muchova levelled the match and ran out to a 2-0 lead in the third, as Barty’s player’s box felt her frustrations at this peculiar summertime bog.

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The Australian’s unforced error tally would eventually climb to 12 in the final set – and 37 for the match – as she overcooked her shots and the Czech made balls previously out of reach.

After facing three break points in her last service game, some big-time serving netted Muchova the match.

Although she lamented the “disappointing” result, Barty said overall her Open – her first time back on court since last summer – was overwhelmingly positive.

“Will it ruin the fact we’ve had a really successful start to our season? Absolutely not,” Barty said.

“The sun will come up tomorrow. We go about our work again. You’re either winning or you’re learning. I think today is a massive learning curve.”

Muchova will play American Jennifer Brady for a place in the final, which is now guaranteed to feature a first-time grand slam finalist.

Brady, who faced countrywoman Jessica Pegula, dug deep after losing the first set and the opening game of the last to secure her maiden Melbourne Park semi-final with a 4-6 6-2 6-1 win.

It’s the second time Brady, ranked 22nd, has made the last four at a grand slam after reaching the same stage at the US Open last year, where she pushed eventual champion Naomi Osaka in a titanic three-setter.

“I’m excited and it will be great to play in front of people, especially the semi-finals,” Brady said.

“In New York, it was an empty stadium so it will be a new atmosphere for me to play the semi-finals of the grand slam in front of fans and I’m really looking forward to it.”

Crowds to be welcomed back from Thursday

Although details are yet to be finalised, Victorian Premier Daniel Andrews has strongly hinted spectators will be back in the stands for the final four days of action at Melbourne Park.

Eagle-eyed fans already noticed that tickets for the semi-finals and finals – some going for more than $300 a pop – were once again available on the tournament’s official ticket-selling platform.

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Speaking to reporters, Mr Andrews said crowd numbers – already well below the imposed 30,000 person cap during the opening week of the tournament – may be reduced further as the state prepares for loosened restrictions.

“They already were reduced but may have to be reduced further. That matter will be resolved in the next few hours,” Mr Andrews said.

“That advice will be out there for people as soon as possible.”

Fans were present in the stands up until Friday, with locals whipped into a frenzy as mercurial Australian Nick Kyrgios was downed by third seed Dominic Thiem on John Cain Arena in a five-set thriller.

However, Novak Djokovic and Taylor Fritz’s third-set epic was halted as security guards ushered fans out of Rod Laver Arena at 11.30pm, before the five day “circuit-breaker” lockdown commenced at 11.59pm.