Former Wimbledon champion Petra Kvitova won the battle of the Czech left-handers to reach the semi-finals of the Sydney International on Wednesday.
The second seed overcame world number 27 Lucie Safarova 7-6 (7/4), 6-2 for her 300th career win to advance to the last four, where she will face Bulgarian Tsvetana Pironkova.
American Bethanie Mattek-Sands, who upset top seed Agnieszka Radwanska, retired with a back injury while trailing fellow American Madison Keys 3-2 in their night quarter-final.
Keys will face German fifth seed Angelique Kerber, who ousted Spaniard Carla Suarez Navarro 6-2, 6-4.
It was only the second meeting between Kvitova and Safarova, who are Fed Cup teammates.
"Lucie's a good friend of mine. We know each other well on and off court because we practised a lot together in the Fed Cup team," Kvitova said.
"So I exactly know what she can play. I knew what she will try, and I try to do my best there."
Kvitova thrashed American Christina McHale 6-1, 6-0 in her opening match and is in a better frame of mind than last year, when she was a first-round loser to Slovakia's Dominika Cibulokva.
"I'm feeling much better than last year and the results show it," she said.
"I was really glad I started here better than last year. I didn't have really nice memories from my last year here."
Kvitova, who beat Maria Sharapova for the Wimbledon crown three years ago, is the highest seed left in the Sydney draw following Tuesday's defeat of Agnieszka Radwanska.
"My serve was great today. It helped me a lot. She's a lefty so it's tricky, too. It's tough to play her for sure," she said.
Pironkova, a semi-finalist at Wimbledon in 2010, upset third seed Sara Errani of Italy 7-6 (7/2), 6-3.
Mattek-Sands said she withdrew to nurse her body through to next week's Australian Open and beyond.
"I've been through my share of injuries. Once you start compensating too much on the court other things start hurting," she said.
"For me, I want to take care of my body. It's not even just about Australian Open, it's about the rest of my year."