OT: that alien scenario should take into consideration not only surface but technology. Who knows what the aliens would choose to play on or with? Who would all you Rafa fans choose then? I know I wouldn't be choosing Rafa on fast grass (or any other surface for that matter) with wooden racquets (the way most greats of the game of tennis have played).
The Aliens can play with whatever technology they have available to them.
We will be playing with current technology. In selecting your player, assume that the current technology has always been the current technology. In other words, assume that if you were to select Borg to play against the Alien that he would have bee playing his entire life with today's technology.
It has to be done this way for purposes of the hypothetical, because it would be ridiculous for us to employ anything less than the best technology when the fate of the world is at stake.