Fourth seeded Caroline Wozniacki eased into the quarter-finals on Wednesday while top-seeded Sara Errani crashed out of the WTA Tour New Haven Open, the final tune-up for the US Open.
Czech Petra Kvitova, seeded third, also booked her spot in the quarter-finals of the $US690,000 ($A775,000) hardcourt tournament.
Four-time champion Wozniacki beat Italian qualifier Karin Knapp 6-1 7-5 while 2012 French Open runner-up Errani was stunned by Russian Ekaterina Makarova 7-5 6-1 at the Connecticut Tennis Center at Yale University.
"She's not a comfortable player for me, as I had lost the last couple times we played," Makarova said. "So today, I was trying to be extra aggressive on my serve and go to the net. I also tried to attack her second serve."
The former world No.1 Wozniacki captured the New Haven event four years in a row from 2008-11.
"I just wanted to try to get through this match in two sets, as I have had a bit of a cold the whole week and felt a little dizzy out there," Wozniacki said. "She really stepped it up and started to play really well in the second set."
Kvitova, who won the 2011 Wimbledon title, outlasted American qualifier Alison Riske 7-6 (7-5) 4-6 6-3.
Errani's loss makes this the first time in New Haven history that neither of the top two seeds advanced to the quarters. Second seed Angelique Kerber of Germany was eliminated on Tuesday.
Makarova's quarter-final opponent on Thursday will be red-hot Romanian Simona Halep, who came from behind to beat Spaniard Carla Suarez Navarro 3-6 6-4 6-1.
The 21-year-old Halep is aiming for her fourth title in seven tournaments and is playing some of her best tennis heading into the US Open which begins on Monday in New York.
Sixth-seeded American wildcard Sloane Stephens slipped past German Julia Goerges 7-5 2-6 6-1 in a night match.
