The players that bother Serena tend to be athletic like Jelena Jankovic at her best she gave Serena a lot of problems because she had the speed and the power. I know Serena and Jankovic have a close head to head I think Serena leads her 5-4 or something like that.
Wozniacki beat Serena last year in Miami but she was off her game because she is very quick and has some power. The other women have to hope Serena is somehow off her game making a lot of errors.
If you have noticed the women who tend to beat Serena are fast, a slow player is not going to get the job done.
Also, someone like Nadia Petrova or Samantha Stosur give Serena a lot of trouble because they can serve.
The problem for the WTA most of the female players cannot serve big like Serena Williams.