John McEnroe believes Serena Williams would struggle to be in the world's top 700 if she played on the men's circuit.
The seven-time Grand Slam champion says that all for her dominance in the women's game, Williams would not be able to transfer that level of success playing against male players.
"If she played the men's circuit she'd be, like, 700 in the world," McEnroe told US radio station NPR.
"That doesn't mean I don't think Serena is an incredible player, and I suppose anything's possible; maybe at some point a women's tennis player can be better than anybody.
"I just haven't seen that in any other sport, and I haven't seen it in tennis. If she had to just play the men's circuit, it would be an entirely different story," added the American.
BBC World Service Sport tracked down the male player ranked 701st in the world – Dmitry Tursunov – and asked him if he felt confident of beating Williams.
"I would hope that I would win against Serena," said Tursunov, a player who was once ranked as high as 20th.
The Russian said that McEnroe wasn't trying to be critical of women's tennis, but that he was simply taking the physical realities of the sport into account.
"It would be a similar argument to: who would run faster, the fastest woman or the fastest man? Tennis is becoming more and more a physical sport, so it's going to be hard for a woman to beat the men," said Tursunov.
"It's not black and white, there are lots of factors to take into account. Physically I might not be in the best shape of my life but as an overall package I'm much better than my ranking would suggest. She is pregnant, and I'm not.
"I've never heard John say anything absolutely stupid – he knows his stuff. What he said about her being an incredible player is correct – explosive, powerful and she puts in a lot of work. But I would hope that I would win."