Sharapova had assured herself of the top spot with a win in the French semifinals. She then went on to complete the career Grand Slam by beating Sara Errani 6-3, 6-2 in the final Saturday.
Sharapova was first ranked No. 1 for a week in August 2005 and then for six weeks in September and October that year as she and Lindsey Davenport switched places several times.
Her third time at No. 1 began Jan. 29, 2007, and lasted seven weeks. She reached the top spot again May 19, 2008, when Justine Henin abdicated the top spot with her first retirement. Sharapova stayed No. 1 for three weeks.
This week marks her 18th week overall at No. 1, tying her with Jelena Jankovic for 15th most.
Sharapova has 9,490 rankings points, which knock Victoria Azarenka to No. 2 at 8,800. Agnieszka Radwanska (7,230) and Petra Kvitova (6,895) remain the No. 3- and 4-ranked players, respectively.
Samantha Stosur, a French Open semifinalist, goes to No. 5 this week with 6,180 points while Williams (5,700) drops one spot to sixth after being ousted in the first round.
Caroline Wozniacki jumps two spots to No. 7 with 4,366 points and Marion Bartoli (4,070) is still eighth. Angelique Kerber (4,055) reached a career high with a No. 9 ranking and Errani (3,350) makes her first appearance in the Top 10 at No. 10.
Li Na, who won the 2011 French Open but went out in the fourth round this year, falls from seventh to No. 11 with 3,245 points.