Australian world number one Ashleigh Barty continued her quest to become the first home champion in 44 years with a straight-set win over Amanda Anisimova.
Barty, 25, has still not dropped a set in Melbourne this fortnight after a 6-4 6-3 win over Anisimova, who knocked out defending champion Naomi Osaka.
Barty will play another American, Jessica Pegula, in the quarter-finals.
Pegula beat fifth seed Maria Sakkari, while two-time champion Victoria Azarenka lost to Barbora Krejcikova.
Greece's Sakkari was beaten 7-6 (7-0) 6-3 by former quarter-finalist Pegula.
Azarenka, who won the title in 2012 and 2013, fell to French Open winner Krejcikova 6-2 6-2.