Williams most recently won the 2017 Australian Open in January after defeating her sister Venus 6-4, 6-4 to win the event for a record seventh time.
In the process, she also overtook Steffi Graf for most Grand Slam singles titles in the Open era with 23 and is one major title away from tying Margaret Court's record of 24.
While some would regard her as the greatest of all time regardless of gender, McEnroe however, believes it is very important to make the distinction.
"Well because if she was in, if she played the men's circuit she'd be like 700 in the world," McEnroe said when asked why gender needed to be included, as quoted by The Washington Post. "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."
"And on a given day, Serena could beat some players. I believe because she's so incredibly strong mentally that she could overcome some situations where players would choke 'cause she's been in it so many times, so many situations at Wimbledon, The U.S. Open, etc. But if she had to just play the circuit — the men's circuit — that would be an entirely different story."
Interestingly enough, the Williams sisters both made a bold claim as teenagers that they could defeat any male player ranked outside the world's top 200 during the 1998 Australian Open.
203-ranked Karsten Braasch went on to challenge and comfortably defeat the duo in single set games, beating Serena 6–1 and Venus 6–2.
Serena was also hesitant in playing against Andy Murray in a charity match when asked about the prospect four years ago.
"For me, men's tennis and women's tennis are completely, almost, two separate sports," Williams said, as quoted on USA Today. "If I were to play Andy Murray, I would lose 6-0, 6-0 in five to six minutes, maybe 10 minutes."
"No, it's true. It's a completely different sport. The men are a lot faster and they serve harder, they hit harder, it's just a different game. I love to play women's tennis. I only want to play girls, because I don't want to be embarrassed."