John McEnroe wasn’t being racist when he said “Serena Williams is the greatest female tennis player in history.”

He was then pressured by the interviewer to say why he qualified the praise by specifying greatest “female” tennis player, rather than calling her simply the greatest tennis player in history. Reluctantly he said “I can’t say that; if she played against men she’d be about 700 in the world.”
You can call that sexist if you want; but it is clearly not racist, as it was her gender being discussed. I don’t think it is sexist either: it just recognises that men in general are bigger, stronger, faster and have more endurance than women. It was no insult to Serena Williams and she should not have taken it as such.
This is exactly why we have separate teams and competitions for men and women.