Teenage Czech Linda Fruhvirtova beat fellow countrywoman Marketa Vondrousova 7-5, 2-6, 6-3 to go to the quarterfinals.
Fruhvirtova, who is 17 years and 273 days old, is the youngest woman still in the competition.
Fruhvirtova, competing in only her second Grand Slam main draw, displayed incredible composure by coming back from a 3-1 deficit to win the match.
After defeating No. 2 seed Ons Jabeur in the previous round in Melbourne, Vondrousova appeared poised for a deep run, but she has since failed to find the form that took her all the way to the 2019 Roland Garros final.
However, the 82nd-ranked Fruhvirtova had other ideas, and she kept her composure in the deciding set to set up a match with the Croat Donna Vekic. Fruhvirtova won her first WTA championship in Chennai in September.