08/09/2013 02:12 EDT | Updated 10/09/2013 05:12 EDT

Williams advances, Cirstea upsets Kvitova at Rogers Cup women's tournament

TORONTO - Serena Williams continues to mow down her opponents at the Rogers Cup women's tournament.

The top-seeded American eased past Magdalena Rybarikova of Slovakia 6-1, 6-1 on Friday for a spot in the semifinals.

Williams has yet to be challenged through three matches, dropping only a combined 10 games in the process. She blasted seven aces past the outclassed Rybarikova and wrapped up the match in a tidy 60 minutes, five seconds.

"Yeah I'm definitely feeling pretty good, playing much better than I have the past month I would say," said Williams. "I'm glad I'm getting back to the feeling and getting into some rhythm."

Rybarikova, who advanced to the quarter-finals when reigning Wimbledon champ Marion Bartoli retired from their match with an injury, was thoroughly dominated but managed to avoid bagels.

Williams, a two-time Rogers Cup champion, is looking to add to her seven titles this season, including a win last month in Sweden. She's sure to give her next opponent, three-seed Agnieszka Radwanska, all she can handle.

Radwanska advanced Friday with a 7-6 (1), 7-5 victory over fifth-seeded Italian Sara Errani, but has never beat Williams in five of their meetings, including last year's Wimbledon final.

"It's a good matchup," said Williams. "She does everything so well. She's playing better too actually so she's having a much better year. It's going to be an interesting match. It's definitely not going to be easy.

"I can only hope that I play well and I can only hope that I come out on top."

Meanwhile, the upset of the day belonged to unseeded Romanian Sorana Cirstea who knocked off defending champion Petra Kvitova with a 4-6, 7-5, 6-2 decision.

Kvitova struggled mightily with her service game. The Czech double-faulted 10 times, her ninth coming on a break point for Cirstea to open the third set 1-0.

"The beginning of the second set I started to feel a little bit low of the energy," said Kvitova. "The serve was really bad after this, and I didn't find energy from my legs."

Kvitova pointed to her late third-round match against Australia's Samantha Stosur as the reason she was sluggish on the court.

"Unfortunately when I finished so late the match here, I didn't sleep well after this," said Kvitova. "Sleep is, for me, it's very important ... for my recovery."

Cirstea, ranked No. 27 in the world, used the momentum from Kvitova's unforced errors to match her best result of the season with a semifinal berth. The 23-year-old Cirstea got past former top-ranked players Jelena Jankovic and Caroline Wozniacki to reach the quarter-finals.

"It's probably (my) best tournament in a long, long time," said Cirstea. "It might come as a surprise to many people ... I don't think for me and my team it's such a big surprise because we have been working consistently on things."

She'll next take on fourth-seeded Li Na of China, who defeated Dominika Cibulkova of Slovakia 7-6 (1), 6-2.

Li, last year's finalist, needed 96 minutes to get by the world No. 20-ranked Cibulkova, who came into the tournament on a roll with a recent title victory over Radwanska at Stanford, Calif.

Li broke to make it 6-5 in the first set, taking the game with a blazing cross-court winner, but couldn't hold and needed a tiebreak to win the set. She cruised through the second set en route to her first semifinal berth since losing at Stuttgart in April.

"I mean, after (the) first I was thinking about, 'OK, one set in the pocket,' so (I was) feeling more confident of course," said Li.

Radwanska's win was a measure of revenge after being eliminated in the quarter-finals by Errani at this year's French Open.

Radwanska and Errani traded breaks virtually the entire first set before Radwanska took the tiebreaker after a smooth volley drop and wide return from Errani on set point. The two players again traded breaks in the second set until Radwanska held serve at 6-5 when Errani returned long on double match point.

Radwanska closed it out in just over two hours, avoiding any repeat of their 2012 meeting at the WTA Championship — an epic three-and-a-half hour, three-set victory for Radwanska.

The 24-year-old from Krakow has played well against Errani with seven wins in nine meetings. Her only loss prior to Roland Garros was in 2006 at Budapest.

There are no Canadians left in the singles draw, but Ottawa native Gabriela Dabrowski and Toronto's Sharon Fichman advanced to the doubles semifinal with a 6-7 (4), 6-2, 10-5 win over the No. 1-ranked doubles team of Errani and Italian partner Roberta Vinci.

"We knew we had to play big tennis," said Dabrowski. "We couldn't go out there and hope for them to make any mistakes. It was imperative today that we kept going after the ball and pressuring them."

In other doubles play, Jankovic and Katarina Srebotnik of Slovenia ousted Americans Raquel Kops-Jones and Abigail Spears 6-2, 6-4, while Anna-Lena Groenefeld of Germany and Czech Kveta Peschke won 7-5, 6-2 over Julia Goerges of Germany and Czech Barbora Zahlavova Strycova

Dabrowski and Fichman will meet Jankovic and Srebotnik in the semifinal.