Other than having the stamina to get through a match without crumbling, Tennis is mainly a game of skill rather than raw athleticism. especially in the women's game where they rely less on powerful serves, etc.
The top male tennis players, whilst obviously very fit, have relatively "normal" body compositions. They spend a lot more time practicing elements of the game itself than doing pure cardio or getting "gym buff".
Compare this to the Olympics which is more of a display of raw strength, speed and endurance in the track and field events... less skill, more pure body conditioning.
The reason Williams dominates is because she has a naturally more masculine body than most women. The amount of "gym work" most women would have to put in to achieve her body shape would mean no time for practicing tennis. And, for whatever reason (this is not sexism, just fact) men are better at sports than women. Williams may dominate the female game but she would be out of the male tour very quickly, probably against a fairly low-level player. Is that because she doesn't put in the effort? No. And the same goes for the other female players vs. Williams.