Agnieszka Radwanska fell to third spot.
Also read: Top seeds Sania Mirza-Martina Hingis beaten in Stuttgart Open final
Russian Maria Sharapova, sanctioned for failing a doping test, fell out of the top 10 after Swiss Timea Bacsinszky rose to the 10th place.
In the women's doubles, top pair Sania Mirza and Martina Hingis maintained their joint position with 12045 points each.
The current rankings and point scores are as follows:
1. Serena Williams (America) 8,235 points
2. Angelique Kerber (Germany) 5,740
3. Agnieszka Radwanska (Poland) 5,665
4. Garbine Muguruza (Spain) 4,847
5. Simona Halep (Romania) 4,650
6. Victoria Azarenka (Belarus) 4,530
7. Roberta Vinci (Italy) 3,505
8. Belinda Bencic (Switzerland) 3,330
9. Petra Kvitova (Czech Republic) 3,067
10. Timea Bacsinszky (Switzerland) 3,065.