Yeah, gotta love the McEnroe comparisons. I'm not even some hardcore tennis junkie and even I know McEnroe got plenty of heat from the man over his outbursts. Hell, it was probably half the reason he became such a big name along with his skill, the "bad boy" schtick, the guy cultivated it intentionally and didn't mind giving up the points or cash as penalty. People seem to be pretending now he just got away with it.
Sampras, Agassi, Hewitt, all of those guys ran hot during matches and got called up on it frequently. Who's the young dude now always throwing hissy-fits, Kirgios or something? Last time I checked, dude, last time I checked, major problems with the tennis fuzz over bad sportsmanship stuff.
It's also ignoring this isn't the first time Serena's done this, it's just an extreme example of it. She's pulled the "you're a hater", "this is racism" stuff at the people in the chair previously. Three times as cited by a couple of articles on google here, the common denominator in all being she was losing at the time.
It's just that Michael Jordan hardcore-competitive-person thing. She's a superstar, her performance isn't living up to what she'd wanted at that point in time, so out come the knives.