I remember the racism Serena suffered during a French match against a Belgian player who cheated. The crowd booed Serena even though she was the victim.
The sisters do suffer racism, it's not the burning crosses KKK overt kind but more covert. They are called 'scary', 'aggressive' in order to make them appear less feminine. They are just excellent sports women who have achieved great things despite very humble beginnings.
Tennis is high pressured with a small group of players travelling around the world together for years and there isn't a single top level player who hasn't thrown a strop or bitched about something or someone yet only the Williams (particularly Serena as she's the most successful) and possibly Andy Murray (because some think he's anti-English) get negative labels.
I think there's always a bit of a backlash from some when the first non-White person/people succeed in a traditionally 'white' field. Every little thing they do is scrutinised, they can't get away with the behaviour their fairer counterparts do. If they do something wrong some people say 'ha it's because they're black' or 'see this is why black people shouldn't be involved in xyz' like they're itching for a downfall. It happened in the early years of boxing, baseball, golf and now tennis.
This same thing where a person is scrutinized because of their colour also happens because of sex when a woman begins to dominate in a traditionally male field. She cannot put a foot wrong lest lots of people pipe up to say she fucked up 'because she's a woman' in order to justify keeping other women down. The Williams sisters are facing both sexism and racism.