Williams responded Monday via Twitter to comments McEnroe made during a weekend interview with NPR to promote his latest book.
Williams tells McEnroe, "I adore and respect you," but asks him to "please please keep me out of your statements that are not factually based."
In one of her tweets, Williams wrote: "I've never played anyone ranked 'there' nor do I have time."
She added: "Respect me and my privacy as I'm trying to have a baby. Good day sir."
Meanwhile, the tennis superstar is showing off her pregnancy with a nude photo on the cover of the August issue of Vanity Fair.
She is seen in profile with her right arm covering her breasts and her pregnant stomach prominently on display. The magazine unveiled the cover Tuesday.
Serena announced her pregnancy with Reddit co-founder Alexis Olhanian in April. The magazine reports the couple will be married in the fall after the baby is born. Williams tells the magazine she "did a double take" and her heart "dropped" when she saw a positive test because it came just before the Australian Open, which she ended up winning.
McEnroe, a former player and now TV commentator, said Williams is the best women's tennis player in history - "no question." But when asked whether she was the sport's best ever, regardless of gender, he made clear he didn't think so.
He said he thought Williams could beat some men, "but if she had to just play the circuit - the men's circuit - that would be an entirely different story." He said if she tried to be part of the men's tour, "she'd be like 700 in the world."
On Tuesday, he refused to apologize for his comments.
Williams is taking a break from the tennis tour and plans to return to competition in 2018.
The American has won 23 Grand Slam singles titles, a record in the Open era, which began in 1968.
McEnroe won seven major titles.
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