After Serena Williams fires back on Twitter, John McEnroe declines to apologize

Granderson: McEnroe's comments about Serena disrespectful (1:09)

LZ Granderson reacts to John McEnroe saying that if Serena Williams played on the men's circuit, she'd be "like No. 700 in the world." (1:09)

Serena Williams fired back via Twitter on Monday after John McEnroe said that if she played on the men's circuit, she'd be "like No. 700 in the world."

In an interview Sunday, McEnroe told NPR that Williams has earned the designation as the greatest woman to play tennis, but he wouldn't put her in the category of greatest to ever play tennis.

"That doesn't mean I don't think Serena is an incredible player. I do. But 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," McEnroe said.

"But if she had to just play the circuit -- the men's circuit -- that would be an entirely different story."

McEnroe, who works for ESPN as an analyst, went on to say that maybe at some point a female tennis player could be better than anybody.

"I just haven't seen it in any other sport, and I haven't seen it in tennis. I suppose anything's possible at some stage," he said.

On Tuesday, McEnroe declined to apologize for his comments during an appearance on "CBS This Morning" and said he "didn't know it would create controversy."

He also offered his suggestion to resolve the debate.

"Why don't you combine, just solve the problem -- I'm sure the men would be all for this -- the men and women play together, and then we don't have to guess," McEnroe said.

He estimated he would be ranked around 1,200th in the world if he were playing professional tennis today.

McEnroe has praised Williams in the past. When she won Wimbledon in 2015, McEnroe called Serena "arguably the greatest athlete of the last 100 years." After Williams' US Open victory in 2012, McEnroe said, "You're watching, to me, the greatest player to ever play the game."

Serena has 23 singles Grand Slams and 14 more in doubles. Roger Federer, by comparison, has 18 singles Slams and none in doubles.