Luxilon, I'll try to answer your question with more consideration. I have to say McEnroe would have a chance, because of his net game. He has enough serve to hold his own, and more finesse than either Serena or Vika, IMO. That said, I think he has more finesse than a lot of the current top men, as well. However, he is 54. Of the two, I'd say Serena might get him, even though she's over 30. I do think she's as competitive as he is.
I'm not sure if he's a terrible set up, or an interesting one, overall. What if you put up McEnroe...one set...against a lot of the "up-and-comers" or otherwise men in the top 30? We've been asking ourselves if these guys have enough nerve to step-up. JMac has loads of nerve, and competitiveness. (Read: "brass ones.") I wonder how many of those guys he might get, for one set?
By the same token, might not the other question be, could Martina Navratilova beat Serena or Vika? If you're just comparing greats from a different era....