I don't think there is a huge difference between the two languages, but I don't speak either so we need an expert to help us out here. Crisstti, where are you?????
Because Rafa's English is heavily accented, he doesn't get enough credit for his linguistic abilities. Roger is lauded for speaking four languages but both Novak and Rafa speak even more. Novak I think is more bold about trying to speak various languages in public but Rafa I believe understands more than all of them but hesitates to speak them.
Rafa is often interviewed on French tv, for instance, and they prattle on in French and he understands them but is shy about responding until this past summer when he made a determined effort to converse more in French. Same with Italian. Both languages are similar to Catalan which is, of course, his native language. I wouldn't be at all surprised if Portuguese falls into much the same category.
Rafa speaks Mallorquin, a dialect of Catalan as Swiss German is to German, Catalan, Spanish, English, French, Italian, and now Portuguese. Pretty impressive and it certainly puts us unilingual North Americans to shame.