Retired tennis star John McEnroe got himself in a bit of trouble with the social justice warrior set this week when he described tennis star Serena Williams as a great “female” tennis player, and went on to say she would only rank “like 700” if she were playing against men.

McEnroe has been making the rounds to promote his new book, and recently appeared on NPR’s Weekend Edition, where the aging pro was asked his thoughts about women’s tennis phenom Serena Williams:

He called Williams a “female player” as he praised her for being the best the world has to offer. That set off the social justice warrior alarm bells with the liberal interviewer.

“Why say female player?,” NPR talker Lourdes Garcia-Navarro asked.

Then McEnroe veered into trouble with the left:

“Well because if she was in, if she played the men’s circuit she’d be like 700 in the world,” McEnroe said.

“You think so?” Garcia-Navarro asked.

“Yeah. That doesn’t mean I don’t think Serena is an incredible player. I do, but the reality of what would happen would be I think something that perhaps it’d be a little higher, perhaps it’d be a little lower. And on a given day, Serena could beat some players. I believe because she’s so incredibly strong mentally that she could overcome some situations where players would choke ’cause she’s been in it so many times, so many situations at Wimbledon, The U.S. Open, etc. But if she had to just play the circuit — the men’s circuit — that would be an entirely different story.”

As Buzzfeed noted, liberals went wild on Twitter to attack McEnroe.

Amusingly enough, Williams herself once said that women can’t really compete against men.

In 2013 ,Williams told David Letterman that she would probably lose if pitted against champion Andy Murray.

“No, it’s true. It’s a completely different sport. The men are a lot faster and they serve harder, they hit harder, it’s just a different game. I love to play women’s tennis. I only want to play girls, because I don’t want to be embarrassed,” Williams said then.

Sounds pretty much like Williams vindicated McEnroe on his point.

Still, Williams piped up today to scold the elder champion for daring to talk about the biological differences between men and women.

McEnroe returned fire at the critics, though, by refusing to apologize to Williams for his comment. With a bit of self-mocking, he said that he is now so old and out of shape that if he tried to go back to the court he’d do rather badly. Indeed, he joked that even if he went into the women’s league he would only rank about “1,200 in the world.”

He also joked that maybe Serena would rank at number five in the world.