Petra Kvitova reached the Adelaide International quarter-finals on Wednesday after her opponent retired and was joined in the last eight by Olympic champion Belinda Bencic and Danielle Collins.
Two-time Wimbledon champion Kvitova saved three set points before claiming the opening set 7-6 (8/6) against China's Zheng Qinwen, who then retired with a left thigh injury.
It was the fourth win from four matches so far this year for the 32-year-old Czech as she prepares for next week's Australian Open, where she made the final in 2019.
"Of course this gives me some good confidence," said the world number 16, who will now play Daria Kasatkina after the fifth seed beat Barbora Krejcikova in straight sets.
"Still a few things to improve, of course, as always, but otherwise I'm happy."
American Collins, who lost to Ashleigh Barty in last year's Australian Open final, defeated Jil Teichmann 6-3, 7-6 (7/2) to also seal a berth in the final eight.
But it took her seven match points to break down the Swiss qualifier and set up a clash with world number nine Veronika Kudermetova.
Fellow Swiss Bencic progressed with a straight-sets win over Anna Kalinskaya.
Bencic is now 3-1 for the year, having split her matches at the United Cup before defeating Garbine Muguruza in the opening round at Adelaide.
The world number 13 has dropped only two sets -- both to world number one Iga Swiatek -- in her last eight matches and is in buoyant mood ahead of the opening Grand Slam of the year.
"I'm absolutely ready," said Bencic, who won gold at the Tokyo 2020 Olympics.
"I'm really strong in the most important matches and in tight moments, so I definitely feel very confident about that. The more you win in those moments, the more it helps."
Spain's Paula Badosa and Brazil's Beatriz Haddad Maia also went through.
In the men's draw, British youngster Jack Draper advanced to the quarter-finals with a 6-4, 6-4 win against American Tommy Paul, while Spain's Alejandro Davidovich Fokina beat home favourite John Millman 6-3, 6-3.
Another Spaniard, Roberto Bautista Agut, and South Korea's Kwon Soon-woo also won.