He certainly would have been good with his talent as would nadal but you have to consider that the talent pool in soccer is way larger. A tennis career from age 5-18 costs more than half a million dollars (coaches, travel...) so it is still a rich mens sport.
and we all know that most superior athletes come from a poor background (like all the runners, basketball and football players...). this is because there are more of them but also because they have more need to succeed. probably there are a lot of soccer players out there who would have the talent to be better tennis players then fed and nadal.
Maybe fed would have been as good as messi but the chance is equally high that he would only be a bench striker in some first league club. I'm not sure if his speed is enough to be as good as messi or ronaldo. he is super quick and moves great but we would need to see how fast he is in a 60 feet sprint.