Williams defeated Jelena Jankovic 6-4, 7-5 Sunday in what was a battle for the tournament championship of the year's final Grand Slam event and the No. 1 ranking.
She is the fifth woman to be ranked No. 1 this year. Justine Henin held the top spot until she suddenly retired from competitive play in June. Maria Sharapova inherited the top spot but was quickly supplanted by Ana Ivanovic. Jankovic also briefly was No. 1 but Ivanovic returned to the top spot before Williams knocked her out of it.
It is the second time Williams, who turns 27 Sept. 26, has been ranked No. 1. She held the top spot from July 8, 2002, until Aug. 10, 2003.
Jankovic remained No. 2 in the rankings while Ivanovic dropped to third, where Williams was two weeks ago. Elena Dementieva moved from sixth to fourth and Dinara Safina, seventh in the previous ratings, is fifth.
Sharapova dropped from fifth to sixth and is followed by Svetlana Kuznetsova, formerly fourth; Venus Williams, who retained the No. 8 spot; Vera Zvonareva, who had been 10th, and Agnieszka Radwanska, who had been 11th.