I was only a baby/child through a lot of Steffi Graf's career, so Serena Williams is easily the best female tennis player I've seen. But when you compare their records it's far too close to say that one is unequivocally better, so to claim racism or - even more ridiculously - sexism either way is, well, ridiculous.
It's not like it is clear cut like Bradman as the GOAT Test batsman - and even then some people (Indians) claim otherwise.
This is what I was getting at.
Serena is clearly in the top few players of all time but you can make the case that Steffi Graf was better. You could also make a case that Williams plays (and wins) a lot of doubles so that should be taken into consideration etc. Either or, it's not clear one way or the other - and that's just two top players from the open era.
Claiming sexism is bizarre considering she's only ever going to be compared to other women. She wouldn't get a game off Djokovic, Murray, Nadal etc. so if she's chasing true GOAT status she's kidding herself. Women's tennis is weak overall. The top women fly through the majors with 6-1 6-0 wins in under an hour whereas guys like Djokovic are fighting out tie breaks, 4/5 setters as early as the 2nd or 3rd round.
https://en.wikipedia.org/wiki/Karsten_Braasch
Braasch competed in a "Battle of the Sexes" contest against the Williams sisters (Venus and Serena) at the 1998 Australian Open when he was ranked 203. A decade and a half older than the sisters, Braasch was described by one journalist as "a man whose training regime centred around a pack of cigarettes and more than a couple bottles of ice cold lager". He nonetheless defeated both sisters, playing a single set against each, beating Serena 6–1 and Venus 6–2.
Claiming racism is a bit different because she is a black woman competing in what has historically been a white-dominated sport but I really don't think there are that many people in tennis circles who downplay her achievements because she is black. I'd say more people dislike her or place a qualifier (tennis pun not intended) on her performances because she and Venus are seen as so much more physically powerful than other women which is a bit unfair as Venus is the bigger, stronger sister and has been less successful. There is a trend towards taller, stronger women in women's tennis anyway. You can call it "racism" (it isn't) but her 'I'm the best and if you disagree you're a racist, sexist hater' attitude also puts people off.