Connecticut - Four-time reigning champ Caroline Wozniacki and second seed Petra Kvitova advanced to the quarter-finals of the WTA's New Haven Open on Wednesday with straight-set victories.
Kvitova, last year's Wimbledon runner-up, easily defeated American Nicole Gibbs, 6-2, 6-4, to advance to Thursday's quarters.
She will next face eighth-seeded Czech Lucie Safarova, who crushed China's Zheng Jie, 6-4, 6-0.
Former world number one Wozniacki, who is seeded third and seeking her fifth straight title at the event, improved to 19-0 lifetime in New Haven with a 7-6 (7/4), 6-2 victory over Sweden's Sofia Arvidsson.
The hardcourt tournament, at the Connecticut Tennis Center at Yale University, is serving as a tune-up event for next week's US Open.
Denmark's Wozniacki defeated Czech Petra Cetkovska in last year's New Haven final. Her quarter-final opponent on Thursday will be Slovakia's Dominika Cibulkova.
Sixth-seeded Cibulkova won 6-4, 6-1 over German Andrea Petkovic, who has battled injuries for most of the year.
This was Petkovic's first tournament in nearly four months and only her fourth event of the season. She entered 2012 ranked 10th in the world.
Petkovic says it is going to take some time to get her match conditioning back.
"I'd play three or four good shots then miss horrible shots I'd never miss in a hundred years," Petkovic said. "My strokes and movement are just fine, but you can never simulate a match in practice, so when the stress of a match comes I feel like it's a totally new situation.
"And I've basically been out for eight months, so really the inconsistency in rallies and matches is the price I'm paying.
"Five or six years ago it was the same thing. I was out eight months for my ACL and when I came back, I'd play a few good matches then lose horrible ones. I just have to stay positive. It's going to take time to get back to my best."
Cibulkova said she was ready to face Wozniacki, who leads their head-to-head series 7-3.
"We know each other really well," Cibulkova said. "She's a year younger but we travelled to all the same junior tournaments and played many matches against each other.
"I know the way she's playing, and she knows how I'm playing. It's two different styles as well -- she's the defensive player and I'm the offensive player going for my shots. It's going to be a really good match."
Results on Wednesday from the $637 000 WTA New Haven Open (x denotes seed):
Caroline Wozniacki (DEN x3) bt Sofia Arvidsson (SWE) 7-6 (7/4), 6-2
Dominika Cibulkova (SVK x6) bt Andrea Petkovic (GER) 6-4, 6-1
Lucie Safarova (CZE x8) Zheng Jie (CHN) 6-4, 6-0
Petra Kvitova (CZE x2) bt Nicole Gibbs (USA) 6-2, 6-4