John McEnroe can't seem to get Serena Williams off his mind.

McEnroe told NPR if the 23-time Grand Slam champion played on the ATP Tour, "she'd be be like 700 in the world."

Watch: McEnroe vs. Murray

The former No. 1, who is promoting his new book, But Seriously also said "on a given day, Serena could beat some (ATP) players."

"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," McEnroe told NPR. "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."

Though Donald Trump once offered to promote a McEnroe vs. Serena battle of the sexes exhibition, McEnroe told NPR he "didn't really want to do it."

"Well I've thought about it," McEnroe said. "I didn't really want to do it, personally. I don't know, people always seemed — I would say why don't they go ask Roger Federer? Or someone, you know they added the old fart that's you know 25 years over the hill.

"And I think I can still play and I think I could still — I mean my kids don't think I can beat her anymore. Maybe I should get her now because she's pregnant, but the truth is that I think that sometimes —I don't know why in tennis, I get it's that one battle of the sexes when Bobby Riggs played Billie Jean."

Of course, this isn't the first time McEnroe has assessed Serena's status in a hypothetical match vs. a male player.

In this 2015 interview with Jimmy Kimmel, McEnroe, who has hit with Williams,  said he could beat the former world No. 1 today.

"I believe I could still take her," McEnroe told Kimmel. "Fifteen years ago, Donald Trump, who seems to be in the news a little bit... he's going to be our President, he made an offer that I felt was not enough...

"Serena, she's got a lot to lose by losing to an old fart like myself. And I've got a lot to lose because if I lose to, God forbid a woman, then I'm not allowed in any men's locker room for the next 15 years or possibly the end of my life, so that's part of why it's intriguing."