Funny you mention the King vs Riggs match as an example. In her prime she beat someone way older that was 30 yrs removed from playing. In fact most thought she didn't even play well and if Riggs was fit he would've won.
How did the Williams sisters do against any man ranked 200 below? Yes, that's as far as they dared challenge and they lost.
In the sport I played competitively, any man ranked in the top 200 easily beat the top women and that is a fact, since in many tournaments men do play against women. I wouldn't even say this sport was one where muscle was a hugely deciding factor.