First, Serena Williams wins the Australian Open out of shape and ranked No. 87 in the world. Then Venus wins Wimbledon ranked in the 20s after not getting past the quarters of a Grand Slam since she won Wimbledon two years before.
Your point about Roger Federer and Althea Gibson is absolutely ridiculous. Your allowances for Federer's background are the end of the story. Federer is not black, not American, not female and is 26 years old. In the rest of the tennis world (which is not so hung up on who is black and who isn't, unlike the States), Althea Gibson is irrelevant to modern tennis (unlike Arthur Ashe, who at least was consistently good and remained a servant to the game).