World No. 1 Ash Barty has been knocked out of the Australian Open by Karolina Muchova in three sets 1-6, 6-3, 6-2 on Rod Laver Arena.
After winning the first set comfortably, racing into a 5-0 lead, Barty had no answer to Muchova in the second or third sets, after the Czech player had recovered from illness to resume the match.
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Barty took an early second set lead, going a break up, before 27th-ranked Muchova went off for a medical timeout.
She returned after ten minutes refreshed and immediately broke back, aggressive and closing in on the net.
The Australian double faulted at 3-4 and Muchova grabbed the initiative to win the set.
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Momentum swung in favour of Muchova as she broke her opponent in the opening game of the third set and stayed the course to cause a major upset.
Muchova will face either Jennifer Brady or Jessica Pegula in the semi-finals.