Radwanska was beaten by Li Na in the quarterfinals of the China Open, where she was the defending champion. That resulted in a loss of 750 ranking points.
Williams didn't play in Beijing, withdrawing before the tournament because of the flu, but Radwanska's loss of points placed Williams ahead of her for No. 3 in the rankings.
Li, who lost in the semifinals, switched spots in the rankings with Sara Errani and is now seventh. Errani slips to No. 8.
The China Open finalists -- Victoria Azarenka and Maria Sharapova -- remain ranked No. 1 and 2, respectively. Azarenka has 10,955 points and Sharapova has 9,135. Williams has 7,900 points and Radwanska 7,265. No. 5 Petra Kvitova has 6,595 points.
Only two Top 10 players are entered in WTA tournaments this week.
Azarenka heads for Austria where she is the top seed for the Generali Ladies Linz tournament. Kvitova is the defending champion for the event but isn't entered. Stosur is the No. 1 seed for the HP Japan Women's Open Tennis 2012. Stosur lost to Bartoli in the 2011 final of the tournament.