The women's rankings are a funny old thing. Vika defends her AO crown, and is in the semis of Qatar, defending, and is now overtaken by Serena who only made the AO quarters?
It's not surprising at all really. With the amount of tournaments Serena won last year since Wimbledon, it was only a matter of time that she would get the No.1 ranking tbh. In the Australian Open, last year Serena went out in the 4th round. Even though she lost in the quarters this year she still gained points while Azarenka didn't gain a single point because she was the defending champion.
In the Qatar Open, last year Serena didn't play in that tournament while Azarenka did and she won the tournament. So by winning each match, Serena gained points while Azarenka didn't because she was the defending champion. By making into the semis, Serena gained enough points to be able to overtake Azarenka in the rankings.
And if you look at the rankings, the difference between the top 3 right now(in points) is quite small.