At 17 years and 273 days, Fruhvirtova is the youngest woman remaining in the draw. Playing in only her second Grand Slam main draw, Fruhvirtova showed extraordinary composure to rally from 3-1 down in the decider and beat her more fancied opponent.
Vondrousova has struggled to replicate the form that helped her reach the 2019 Roland Garros final but the 23-year-old had looked set for a deep run in Melbourne after beating second seed second seed Ons Jabeur in the last round.
World number 82 Fruhvirtova - who won her first WTA title in Chennai in September - had other plans, however, and held her nerve in the third set to set up a meeting with Croatian Donna Vekic.