Four-time winner Wozniacki defeated American wild card Sloane Stephens 7-6 (8/6), 6-2 in a quarter-final contest at the Yale University hardcourt facility.

The former world number one Wozniacki, who is seeded fourth this week, captured the New Haven event four years in a row from 2008-11.

The third-seeded Czech Kvitova fought back from a first-set stumble to outlast Russian Anastasia Pavlyuchenkova 2-6, 6-2, 7-5.

The former Wimbledon champion Kvitova topped Russian Maria Kirilenko in last year's New Haven final.

Kvitova lost six more points than she won in her rain-delayed match against Pavlyuchenkova, but came back from 40-15 down in the last game to break her opponent's serve and post the win.

"It's important to have matches like these where you have to fight until the end where you're not necessarily playing your best. That's part of tennis," Kvitova said.

"Of course, I prefer shorter, two set matches, but I won the last point and that's the important thing."

Kvitova is now two wins away from defending a title for the first time.

"I think coming back to a place where you played very well the last year, you have a lot of emotions and you know the pressure is there as defending champion, and you're defending a lot of points," Kvitova said. "But on the other hand, you know that you can play well here. "

In other matches, surging Romanian Simona Halep beat Russian Ekaterina Makarova, 6-1, 7-6 (8/6), while Czech Klara Zakopalova earned 4-6, 6-0, 6-4 triumph over 2009 New Haven runner-up Elena Vesnina.

The 21-year-old Halep, who is seeking her fourth title in seven tournaments, will take on Wozniacki in the semis, while Zakopalova will go up against Kvitova in an all-Czech semi.