In an interview with NPR on Sunday, McEnroe said Williams is the greatest female player in history, but she wouldn't be one of the greatest ever.
“Dear John,” Williams wrote in a statement on Twitter on Monday. “I adore and respect you but please please keep me out of your statements that are not factually based. I’ve never played anyone ranked ‘there’ no do I have time. Respect me and my privacy as I’m trying to have a baby. Good day sir.”
Even though his comments came off as an insult, McEnroe did not apologize for his comments on CBS This Morning on Tuesday.
"I don't want anything to go wrong with Serena because she's pregnant," McEnroe said on CBS This Morning. "I don't want to upset her or whatever it was. I think she was doing it tongue-in-cheek as well, and I think that deep down we're talking about something, I can't even believe we're talking about it."
McEnroe, 58, won seven Grand Slam titles during his career while Williams has, so far, won 23.
Williams, who is more than seven months pregnant with her first child, is also making news for her nude photo shoot for Vanity Fair’s August cover.
Wearing little more than a hip chain and nude underwear, according to Eonline, Williams, 35, showed off her baby bump in a photo strikingly similar to actress Demi Moore's 1991 nude cover shot for "Vanity Fair."
Both photos were shot by famous photographer Annie Leibovitz.
Williams is engaged to Reddit founder Alexis Ohanian, the baby's father, and said in the magazine interview that she had been caught off guard by her pregnancy.
The couple is planning to tie the knot sometime this fall.