John McEnroe is refusing to apologise for his comments about women's world tennis number one Serena Williams - and believes men should play against women to settle the argument.
The former world No.1 stirred up a storm after telling US radio station NPR that Williams, who holds the Open era record of 23 grand slam singles titles, would be ranked around 700 in the men's game.
McEnroe has been widely criticised for appearing to belittle the achievements of Williams, but defended his remarks saying men and women playing against one another would resolve the debate.
After describing Williams on NPR as the greatest female player of all time, McEnroe was asked why he had not simply said greatest player.
"I felt the need, however unfortunately, to defend myself and say what I really felt, which is about what I think she would be," McEnroe said.
"I've got a solution. Solve the problem, and I'm sure the men would be all for this - the men and women play together. And then we don't have to guess."
Williams responded to McEnroe's comments with a pithy response on Twitter.
The 35-year-old, who is away from the tour preparing to give birth to her first child, wrote: "Dear John, 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" nor do I have time. Respect me and my privacy as I'm trying to have a baby. Good day sir."
When she was much younger Williams and sister Venus did however take part in a 'battle of the sexes' against German Karsten Braasch, who was then world No.203.
Braasch played a set against each, beating Serena 6-1 and Venus 6-2.
When McEnroe was asked if he would like to apologise he was emphatic in his response.
"No. I didn't know it would create controversy. I've said this 1,000 times," McEnroe said.
"I don't want anything to go wrong with Serena because she's pregnant, I don't want to upset her.
"I think she's doing it tongue in cheek as well and I think deep down we're talking about something... I can't even believe we're talking about it."
Williams has already made it clear she is determined to return to the tour next year, with Margaret Court's all-time record of 24 slam singles titles an obvious goal.