In an interview with CBS This Morning on Tuesday, McEnroe said he would not apologize for the comments, which he made in an interview with NPR on Sunday.
Instead, McEnore lamented that tennis players are often asked to rank each other, regardless of their gender — which he says athletes in other sports rarely consider.
When asked why he would make the comment, McEnroe said: "It wasn't necessary. I didn't know it would create controversy."
"She's the greatest female player that ever lived," McEnroe said.
McEnroe, a retired tennis star who is now promoting his new book, was given the chance to re-do his rankings on CBS This Morning. He placed Williams in the fifth spot. "OK, you happy now?" he appeared to joke. As for himself, McEnroe said he "would be, currently, about 1,200 in the world." (When he was active in the sport, McEnroe had attained the No. 1 ranking in singles and doubles.)
"Respect me and my privacy as I'm trying to have a baby," Williams, who is pregnant, tweeted. "Good day sir."
Williams has won 23 singles Grand Slam titles, 14 more in doubles and is widely regarded as one of the greatest athletes in the world.
McEnroe's initial comments came as he was responding to a question from NPR about why he doesn't call Williams the greatest tennis player, period.
"If she played the men's circuit she'd be like 700 in the world," he 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."
McEnroe's assessment of Williams's rank among her male counterparts differs from one he gave in the past. In 2015 after her Wimbledon win, McEnroe called Williams "arguably the greatest athlete of the last 100 years," according to ESPN.