I'm no fan of Serena Williams as a person, I think she has an arrogant attitude sometimes, however you cannot dispute that she receives a lot more negative commentary than her white female sportswomen paramours.
I think there's a conflation of both sexism (she has a very strong, not typically eurocentric feminine athletic build) and her skin colour. Tennis has been notoriously elitist, classist, and racially homogeneous (narrow might be a better word) and that's been said by people who professionally comment on and are responsible for recruiting and nurturing young talent in the game. Some have said Britain would have more champions if it was less so.
So when one sees criticism, they look at the content and spirit of it and think is this valid or mean? and would it be said by another white champion? Often the conclusion to those above questions in regard to critique of either of the sisters is yes and very probably not. Because in tennis well as far as Britain goes, I see white champions given god like praise. There was some girl I forget her name now, who won a few years ago and my god, you would have thought saying her name would produce rain the way she was lionised by the media. Offered golden brand deals straight away. It's a big contrast.
I mean belittling her receiving the wildcard: What's that about? This is a woman who has done a ton for female players of the sport (I think she was vocal in arguing female players should receive the same prize money men do, but open to correction on that) inspired generations of women, especially women of non-caucasian ethnicities, and women from poor backgrounds of every ethnicity intimidated by poor access to the sport, and been a prolific champion breaking records. So should she have been offered the WC? Why the hell not? Should she have accepted it? Yes, if she wants to play. Particularly if she had the Olympics in view.
Any person who was a champion and major advancer in any sport should be able to re-enter that sport at a level slightly above in my opinion. If they're not fit they'll be taken out as has happened here. I bet her opponent is on cloud nine to have defeated a legend of the sport and isn't spinning negative narratives in her head.