A newspaper in Australia is catching A LOT of heat over a cartoon that was trying to spoof Serena Williams' incident at the U.S. Open -- because it's racist as hell.

The cartoon -- drawn by Mark Knight (editorial cartoonist for the Herald Sun newspaper) -- shows the tennis superstar stomping on her racket.

But, let's get serious, it looks like a Jim Crow-era, Sambo-style caricature of a black person -- not Serena Williams.

The cartoon has been blasted by athletes and celebs including J.K. Rowling who said, "Well done on reducing one of the greatest sportswomen alive to racist and sexist tropes and turning a second great sportswoman into a faceless prop."

ESPN's Jemele Hill noted the racism is "About as subtle as Fran Drescher’s voice."

So far, it doesn't seem like the Herald Sun is backing down -- they retweeted the image and haven't pulled it.

At least, not yet.

As we reported ... Serena was penalized several times during Saturday's U.S. Open finals match against Naomi Osaka. Serena went off on the judge claiming he was a "thief" who stole a point from her.

The judge then penalized Serena a game for the "verbal abuse." Williams continued to go off on the guy to other tournament officials claiming the judge would never treat a man that way.