Two things to note:
- Richards was born in 1934, and transitioned around 1975, at age 41.
- She was not permitted to compete in professional tennis until 1977, when she was 43.
So, she was playing at a high level, as a woman, in her early 40s. When she reached #20 in the rankings, she was 45 years old.
It’s also important to remember that, while some elite athletes are, today, able to compete at a high level well into their late 30s and 40s, thanks to much better sports medicine and sports nutrition, that wasn’t the case in the 1970s; it was extremely rare for a tennis player to succeed at that level in their 40s, in that era.
As far as I can tell, prior to transitioning, in the 1960s, Richards was a very good men’s player, but to the level of being a high-ranking amateur, and not at an elite professional level – and that was five to ten years before she began playing on the WTA tour as a woman.