Ivanovic was no match for a dominant Williams

Williams, playing her fourth tournament of the year after struggling with a knee injury, needed just 66 minutes to beat 18-year-old Ivanovic.

The seven-times Grand Slam winner faces Amelie Mauresmo in the fourth round.

The world number one made surprisingly hard work of beating Italy's Mara Santangelo 6-3 3-6 6-2.

Former champion Lindsay Davenport was also made to struggle and saved two match points before finally ending the challenge of Katarina Srebotnik.

Srebotnik fought back from 3-1 down in the third set and had two points for a shock win at 6-5 but Davenport finally prevailed 3-6 6-3 7-6 (7-5).

"It was just one of those days where I didn't feel like I was hitting the ball all that cleanly," Davenport said.

The American will play Patty Schnyder in the last 16 after the seventh seed beat Marion Bartoli 0-6 6-3 6-3.

Fifth seed Nadia Petrova and 2005 finalist Mary Pierce both went out in the third round on Sunday.

Petrova suffered her third straight defeat to France's Tatiana Golovin, bowing out 7-5 6-7 (4-7) 6-3.

Pierce, playing in only her fourth tournament this year after struggling with injury, lost 4-6 6-0 6-0 to Li Na.

Li, 24, is the first Chinese woman to reach the last 16 of the US Open where she will face Maria Sharapova, after the Russian beat Elena Likhovtseva 6-3 6-2.

"I had a couple of past meetings with her this year," Sharapova said of Li.

"She is improving each time out and she is great mover on the court. I have beaten her in the past, I just have to go out there and do the same."

Russian Dinara Safina made quick work of Jarmila Gajdosova, beating the Slovakian 6-3 6-0 to set up a fourth-round meeting with Virginie Razzano.