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 four-month layoff because of a right 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 Monday.
Mattek-Sands advanced to play fellow American Madison Keys, who defeated Croatian wild-card entry Ajla Tomljanovic 6-0, 3-6, 7-6 (3).
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 Tuesday.
In men's play at Sydney, seventh-seeded Marin Cilic of Croatia defeated Jan-Lennard Struff of Germany 3-6, 6-3, 6-3 and Nicolas Mahut of France beat American Ryan Harrison 7-6 (2), 7-5.
At the ATP tournament at Auckland, New Zealand, American qualifier Donald Young beat Federico Delbonis of Argentina 7-5, 6-2. German Philipp Kohlschreiber, the fifth seed, and Frenchman Benoit Paire, the sixth seed, were also winners.
At the Hobart International WTA event, Australian teenager and wild-card entry Storm Sanders beat China's Peng Shuai, ranked nearly 200 places ahead of her, 6-2, 6-2. Sanders was only given a wild card when fellow Australian Ashleigh Barty withdrew with a hip injury.
Lleyton Hewitt, meanwhile, withdrew from the Kooyong exhibition tournament in Melbourne, saying he feared playing would harm his preparations for the Australian Open.
The veteran Hewitt, who has struggled with injuries in recent years, reclaimed his place as Australia's top-ranked player when he won the Brisbane International on Sunday with a 6-1, 4-6, 6-3 upset victory over Roger Federer.
"At the moment it's just all about recovery for me," the 32-year-old said. "When you get older it gets a bit tougher than when you were 16."
Most of the top players have chosen to take the week off before the year's first Grand Slam and are training instead at Melbourne Park.