ADELAIDE, Australia (AFP) — In-form Petra Kvitova reached the Adelaide International quarterfinals after her opponent retired and was joined by resurgent Olympic champion Belinda Bencic.

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 No. 16.

"Still a few things to improve, of course, as always, but otherwise I'm happy."

American Danielle 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.

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 No. 13 has dropped only two sets — both to world No. 1 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."

In the men's draw, British youngster Jack Draper advanced to the quarterfinals with a 6-4, 6-4 win over American Tommy Paul, while Spain's Alejandro Davidovich Fokina beat home favorite John Millman 6-3, 6-3.