"She'll be in the top 10 for sure. I think she's pretty close already," McEnroe told reporters at an exhibition in Hong Kong.
McEnroe added that one of the reasons why American women are now succeeding more than U.S. men is because the sport offers women greater economic opportunities.
"For ladies one of the great things about tennis is the opportunity is there for them, more than virtually any other sport they play,” he said. “They're given equal opportunity in terms of prize money, much closer to a fair shake growing up. And that's good and that's why you see a lot more girls in America turning to tennis than say American football, basketball, baseball, the other sports."