Serena Williams of the United States reacts during the third round match of women's singles against Victoria Azarenka of Belarus at US Open tennis tournament in New York, Sept. 3, 2011. Williams won 2-0. [Xinhua]

Williams, the 13-time Grand Slam champion, seeded just 28th, next faces either former world number one Ana Ivanovic of Serbia or U.S. wildcard Sloane Stephens for a place in the quarterfinals.

In a tournament without defending champion Kim Clijsters, and which saw Maria Sharapova, French Open champion Li Na and Wimbledon winner Petra Kvitova all fail to reach the second round, Williams looks the hot favorite.

Asked what it felt like to be blitzed 5-0 after just 17 minutes of their match, Azarenka said: "It's painful."

The Belarusian believes the American will comfortably win the title next weekend.

"Definitely. She's playing at a higher level than I've seen her play in a few years."

Williams, who saw three match points slip away in the ninth game of the second set, insisted she still has much to work on.

"I think in the first set I played some really good tennis. In the second, she kind of dictated. I probably could have played better."

Wozniacki, the runner-up to Clijsters in 2009 and still searching for an elusive first Grand Slam title, made the last 16 with a 6-2, 6-4 victory over America's world number 103 Vania King.

The Dane next faces 15th-seeded Russian Svetlana Kuznetsova, the 2004 champion, who eased past Uzbekistan's Akgul Amanmuradova 6-4, 6-2.

The 2010 French Open champion Francesca Schiavone, the Italian seventh seed, saved a match point to defeat South Africa's Chanelle Scheepers 5-7, 7-6(5), 6-3.

Schiavone next faces Russian 17th seed Anastasia Pavlyuchenkova who beat Serbian 11th seed Jelena Jankovic, the 2008 runner-up, 6-4, 6-4.

German 10th seed Andrea Petkovic made the last 16 for the second successive year with a 6-4, 6-0 win over Italian 18th seed Roberta Vinci 6-4, 6-0.

She will play Carla Suarez Navarro, who put out fellow Spaniard Silvia Soler-Espinosa 6-0, 6-4.

In men's category, eighth seed Mardy Fish defeated South Africa's Kevin Anderson 6-4, 7-6(4), 7-6(3) and next plays French 11th seed JoWilfried Tsonga, who defeated Fernando Verdasco, the 19th-seeded Spaniard, 6-3, 7-5, 6-4.