Top seed and defending champion Angelique Kerber suffered a fourth-round exit from the Australian Open to world number 35 Coco Vandeweghe.
German Kerber, 29, claimed Grand Slam titles at Melbourne and the US Open last year, but lost 6-2 6-3.
American Vandeweghe, 25, faces a last-eight meeting with Spain’s French Open champion Garbine Muguruza, who beat Romanian Sorana Cirstea 6-2 6-3.
Venus Williams also progressed, beating Germany’s Mona Barthel.
The 36-year-old seven-time Grand Slam champion – the oldest player in the women’s singles – won 6-3 7-5 in Melbourne and will take on Anastasia Pavlyuchenkova, who beat fellow Russian Svetlana Kuznetsova 6-3 6-3.