23-time Grand Slam champion Serena Williams has hit back at John McEnroe, saying the tennis legend should respect her and her privacy and should abstain from making comments that are "not factually based".
McEnroe had claimed that Serena, who is the best player of her gender, would be ranked 700 if she plays on the men's circuit.
The seven-time major champion, speaking as part of a tour to promote his new book 'But Seriously', said that while he has great respect for Serena, he would place the 23-time Grand Slam winner "like 700 in the world" on the men's circuit.
"Best female player ever - no question."
"If she had to just play the circuit - the men's circuit - that would be an entirely different story," he added.
However, Williams on Monday hit back at McEnroe and wrote on Twitter, "Dear John, I adore and respect you but please please keep me out of your statements that are not factually based."
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"I've never played anyone ranked 'there' nor do I have time. Respect me and my privacy as I'm trying to have a baby. Good day sir," she added.
McEnroe had also asserted that Williams' mental toughness would be one of the things that she would be having her way on the men's tour.
"The reality of what would happen on a given day is Serena could beat some players, I believe, because she is so incredibly strong mentally," he said.
"She could overcome some situations where players would choke because she's been in it so many times, so many situations at Wimbledon, The US Open, etc," he added.
In April, Williams announced that she was pregnant and would not compete for the remainder of the season.
The American became the oldest woman to win a grand slam when he won the 2015 Wimbledon title at the age of 33.
The 35-year-old tennis star has 23 grand slam singles crowns and 14 doubles titles in majors.