Briton Katie Boulter battled back to knock out American teenager Coco Gauff 3-6 7-5 6-2 to reach the last-16 of the Gippsland Trophy warm-up tournament in Melbourne on Tuesday.
Gauff, the 16-year-old seeded 14th, double-faulted three times in the final game as Boulter sealed a historic victory.
The 24-year-old will face second seed Naomi Osaka in the next round.
'This one sucks' - Osaka upset after shock defeat in Tokyo
Osaka stepped up her preparations for the Australian Open by powering past Alize Cornet 6-2 6-2.
In her first competitive match since winning the U.S. Open in September, the 23-year-old Japanese player sent down six aces and converted all four of her breakpoints to seal victory in 71 minutes.
"I always feel nice here in Australia, I feel like everyone welcomes me," said Osaka, who won the Australian Open in 2019.
"I'm really glad that you guys opened your arms and we're able to play here."
It was a good day for British tennis as fifth seed Johanna Konta eased past Bernarda Pera 6-2 6-3, saving three break points en route to the last-16.
World No.7 Aryna Sabalenka's 15-match winning streak came to an end when she lost 6-1 2-6 6-1 to Estonia's Kaia Kanepi.
Kasatkina beat Polona Hercog 6-4 6-3 to reach the round of 16. Joining her was Caroline Garcia, who downed Timea Babos 6-4 6-4.
French Open champion Iga Swiatek recovered from a set down to beat Slovenia's Kaja Juvan 2-6 6-2 6-1.
The Australian Open will begin on February 8.
Additional reporting from Reuters.
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