Three-time champion Serena Williams was an easy fourth-round winner, while second-seeded Agnieszka Radwanska was sent packing Monday at the U.S. Open.

The fourth-seeded former world No. 1 Williams double-bageled helpless Czech Andrea Hlavackova 6-0, 6-0 in a mere 56 minutes at Ashe Stadium, while 20th-seeded Italian Roberta Vinci surprised the Wimbledon runner-up Radwanska 6-1, 6-4 at Armstrong Stadium on Labor Day in the United States.

Williams was last year's U.S. Open runner-up to Aussie Samantha Stosur.

The 30-year-old 14-time Grand Slam champion Williams captured Wimbledon in July and secured a singles gold medal at the London Olympic Games, also at the All England Club, last month.

The 14-time Grand Slam champion Williams' quarterfinal opponent will be fellow former No. 1 Ana Ivanovic of Serbia. The 12th-seeded former French Open champion Ivanovic handled Bulgarian Tsvetana Pironkova 6-0, 6-4 on the hardcourts at the USTA Billie Jean King National Tennis Center.

The 24-year-old Ivanovic will now appear in her first major quarterfinal since capturing the French Open more than four years ago.

"There were some good moments and some tough ones, but it's a process. I understand it better now," Ivanovic said. "There are times when you realize that it's exactly what it is: It's a process. You can't have everything at the same time. You have to be consistent on practice courts and then in the matches."

Ivanovic is 0-3 lifetime against Williams, including a pair of losses at the U.S. Open. The American beat the Serb in the round of 16 here a year ago and also bested Ivanovic in the third round in Flushing back in 2006.

Surprise French Open runner-up Sara Errani is now a surprise first-time U.S. Open quarterfinalist after ousting sixth-seeded German Angelique Kerber 7-6 (7-5), 6-3 at Armstrong on Day 8.

The left-handed Kerber was a U.S. Open semifinalist last year and reached the Wimbledon semis in July.

The 25-year-old Errani has now reached at least the quarterfinals at three of the four Grand Slam events in 2012.

Errani will face her good friend and doubles partner Vinci in the round of eight on Wednesday.

The 29-year-old Vinci beat Radwanska for the first time in five tries on the WTA on Monday, as the Italian veteran stunned her Polish counterpart on her second match point with an easy forehand winner.

The women's quarterfinals will commence Tuesday, when world No. 1 and 2012 Australian Open champion Victoria Azarenka takes on a seventh-seeded Stosur and third-seeded former No. 1 Maria Sharapova tangles with 11th-seeded Frenchwoman Marion Bartoli. The Russian star Sharapova was the 2006 U.S. Open titlist and completed a career Grand Slam with a win at the French Open in June. Bartoli was a Wimbledon runner-up in 2007.