Williams, 35, is currently six months pregnant. She is on maternity leave for the remainder of 2017, but is still ranked No. 4 in the world. She only added to her legend when she won the 2017 Australian Open while two months pregnant.
Williams is widely considered the greatest women's tennis player of all time.
She has dominated women's tennis, sitting atop the WTA world rankings for more than 300 weeks. Last year she tied Steffi Graff for the most consecutive weeks ranked No. 1 (186).
But McEnroe isn't buying her dominance against men. He isn't even putting her on the same court.
He paid respect to Williams as the best female tennis player in the world, but was asked Sunday on NPR why he qualifies her not being the "best player in the world?"
"Well because if she was in, if she played the men's circuit she'd be like 700 in the world," McEnroe said. "...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."
The 58-year-old McEnroe said that he thinks he could still beat Williams, but his kids "probably don't."
And he's consistent in that thought.
In 2015, he told Jimmy Kimmel the same thing.
"My daughters are over there," McEnroe told Kimmel. "I think it's mixed. They think I'd probably lose to Serena. I believe that I could still take her."
"15 years ago, Donald Trump, who seems to be in the news a little bit...going to be our president in about a year. He made an offer which I felt like was not enough. I think he was humoring the two of us. 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, them 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."
He also told Kimmel he didn't think he could "take" Kimmel in a boxing match.
McEnroe was ranked No. 1 in singles and doubles in his career. He also won seven Grand Slam singles titles. Williams has a record 23 Grand Slam singles titles.