Federer can be a great soccer player but to expect him to be as great as Messi is highly unlikely, let alone better. And Messi wouldn't be a great tennis player(too small) had he chose to play tennis. They both are great in their own sport, however, Federer is more dominant in tennis than Messi is in soccer. Very few athletes were more dominant than Federer(ie Gretzky, Rice)
Federer > Davydenko > Nadal > Federer
Nadalgaenger: "Djokovic is clearly better than Pete"