Let us start from the premise that VEnus and Serena both serve well and play well. No perfect, but pretty good. In that case, Serena has far and away he best chance of beating Venus. No one else is really close. In Serena's case, well, according to Serena, Serena played pretty well the day Chanda beat her. She couldn't serve her way out of a paper bag when Justine beat her, and NOTHING worked the day Patty beat her. Credit Patty though, for giving Serena an endless variety of problems to solve.
Theirs only one player this year that made the Williams sitser work and that was Henin.
5 finals against them. And some were very close. Only the last encounter against Serena was not good at all. Serena is just becoming too good.
In these form nobody will beat her. But the player that make them work in my eyes is Henin. Their is always one tight set and mostly their mathes are 3-setters. She already beat Venus and Serena