The article began with reference to the Williams sisters, and then mentioned one other current female tennis player. Given this I suggest that the issue is not sexism as much as it is racism.
The Williams sisters are attractive black women, with features that are traditionally considered black. The other current female tennis players are white with no real distinguishing features between them.
The point is that tennis watchers would rather see a young nubile white female running up and down the court than see a young nubile black female run up and down the court.