Sydney - American qualifier Bethanie Mattek-Sands defeated top-seeded and defending champion Agnieszka Radwanska of Poland 7-5, 6-3 at the Sydney International on Tuesday.
Mattek-Sands, who is back on tour after a long layoff with a knee injury, lost in the first round in Brisbane last week and had to win three matches just to qualify for the main draw in Sydney.
Both tournaments are tuneup events for the Australian Open, which begins next week.
Mattek-Sands advances to play the winner of the match between fellow American Madison Keys and Croatian wild-card entry Ajla Tomljanovic.
Angelique Kerber of Germany was also through to the quarterfinals after beating Kaia Kanepi of Estonia 6-3 6-4.
Second-seeded Petra Kvitova and Caroline Wozniacki were playing second-round matches later on Tuesday.