World number one Simona Halep crashed to a straight-sets defeat against unseeded Ashleigh Barty in the second round of the Sydney International on Wednesday.
Halep, playing in her first tournament since a back injury cut short her 2018 season, lost 6-4, 6-4 in 87 minutes with the result dealing a blow to her Australian Open preparations.
Australian Barty, who is ranked 15th in the world, fired 26 winners and won four out of seven break points as she recorded her first victory over a reigning world number one.
The 22-year-old Barty faces either Elise Mertens or the winner of the first round clash between Anett Kontaveit and Monica Puig in the quarter-finals.
Top seed Isner beaten by Fritz in Auckland second round
Top seed John Isner suffered a 7-6(3), 7-6(5) defeat at the hands of fellow American Taylor Fritz in the second round of the Auckland Classic on Wednesday.
The big-serving Isner, ranked 10th in the world, fired 26 aces but struggled on his second serve as Fritz took full advantage to edge the match in an hour and 39 minutes.
"I really wanted to get the win on him. I knew I could," said Fritz. "We had a lot of close points and it just feels really good to get through that," world number 50 Fritz said.
The 21-year-old converted one out of three break points and saved three on his serve.
"I like to think that one of my strengths is playing under pressure and playing the big moments well," Fritz added.
"More often than not, the matches I lose, that's where I get let down. So I'm happy to start the year ... playing well under pressure and handling those big moments well."
Fritz will next face Briton Cameron Norrie, who overcame Joao Sousa 7-6(5), 6-4.
American Tennys Sandgren also advanced to the quarter-finals, beating third seed Marco Cecchinato of Italy 6-3, 6-3 to set up a meeting with Leonardo Mayer.
The Argentine saved five out of six break points to move past Matteo Berrettini 5-7, 7-6(5), 6-3.