Serena Williams sent an ominous warning to her title rivals with a 'double bagel' victory over Andrea Hlavackova to reach the quarter-finals of the US Open.

The fourth seed is the favourite to win a fourth US Open crown at Flushing Meadows after her stellar summer that brought victory at Wimbledon and two Olympic gold medals.

Hlavackova and fellow Czech Lucie Hradecka were beaten by Williams and sister Venus in the doubles finals at both Wimbledon and the Olympics.

So the American was certainly not the opponent she would have hoped to see across the other side for her first appearance in singles in the fourth round of a slam.

In the first set Hlavackova was completely outplayed and won only 10 points but she never let her head drop and was close to winning a game on a number of occasions in the second set.

She even had her first break point when Williams served for victory but she could not take it and the 30-year-old wrapped up a 6-0 6-0 win after 57 minutes.

Williams hit 31 winners and made only seven unforced errors to set up a quarter-final clash against Serbia's Ana Ivanovic, who she has beaten in their three previous meetings.

The American said: "She was fighting really hard. If you give people any type of chance, especially in tennis, the match is never over until you shake hands. There is always a comeback available.

"So I didn't want to give her that opportunity. She was getting so pumped up and she never gave up. I thought that was really incredibly positive.

"I feel like I'm getting more comfortable with the court and comfortable with the conditions. I'm getting back to more my game, which is good. I like to play better during the second week. Hopefully I can do that."

Ivanovic's one grand slam title came at the French Open in 2008, after which she fell away to such an extent that this is her first time back in the quarter-finals at any of the sport's four major tournaments since.

The 24-year-old joked after beating Tsvetana Pironkova 6-0 6-4 that if she had known it would have taken her so long to get back to the quarter-finals, she would have gone on holiday for four years.

She said: "It's been like a rollercoaster. There were some good moments and some tough ones, but it's a process. I understand it better now.

"There are times when you realise 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 the practice courts and then in the matches. I felt I had that this year. I have been quite consistent at times.

"I put myself lots of times in opportunities for the big matches and the big wins, and I never managed to do that. Now I have another chance, and I really hope I can pull it together this time."

Italian doubles partners Sara Errani and Roberta Vinci will meet in the quarter-finals after both caused upsets.

Errani ousted sixth seed Angelique Kerber and was followed on to Louis Armstrong Stadium by Vinci, who defeated world number two and Wimbledon finalist Agnieszka Radwanska to reach her first singles grand slam quarter-final.

Both women have made big strides this season both individually and together.

Errani made the quarter-finals at the Australian Open and then went on a brilliant run to the final of the French Open, where she lost to Maria Sharapova.

Vinci reached her first fourth round at a slam in singles at Wimbledon while, as a pair, they made the final in Australia and then won the women's doubles title at Roland Garros.

Vinci, the 20th seed, has made quiet progress through the draw but she certainly made a statement on Monday with a performance full of guile to beat an out-of-sorts Radwanska 6-1 6-4.

Vinci had lost all their four previous matches, and she said: "I'm so happy, so excited. It was a tough match for sure but I think I played a really good game today."

And the 29-year-old does not have too many misgivings about playing Errani, saying: "For sure an Italian goes to the semi-finals so I'm happy to play Sara. She's my best friend. I'm so pleased she won today."

Errani was involved in a closer match against Kerber, whose brilliant year began here 12 months ago with a surprise run to the semi-finals.

But Errani turned the tables, edging a very tight first set and then proving the stronger in the second to win 7-6 (7/5) 6-3.

Errani has credited her turnaround in form largely to her switch from a Wilson racquet to Babolat, and the 10th seed said: "This year with this racquet made me feel different on the court and make me feel much better. This is the best year of my life."