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)

Seven-time major champion John McEnroe, when asked, can confidently say Serena Williams has earned the designation as the greatest woman to play tennis.

However, in an interview Sunday as part of the book tour promoting his new memoir "But Seriously," McEnroe qualified her greatness by telling NPR that if Serena played on the men's circuit she'd be "like No. 700 in the world."

"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 went on to say that maybe at some point a women's tennis player can 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."

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." And after Williams' US Open victory in 2012, McEnroe stated, "You're watching, to me, the greatest player to ever play the game."

At 33, with that 2015 Wimbledon victory, Williams became the oldest woman to win a Grand Slam title, surpassing by 26 days Martina Navratilova and her Wimbledon title in 1990.

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