I'm sorry, but I can't accept the friendly matches for reasons of my shape...
Remarks about the translation
More background is needed!! If "forma" is supposed to mean "minha forma fÃsica", than I would say "because I'm not in a good shape", or "because I want to keep my shape", depending on what the person is trying to say.
I suggest you choose one of your own suggestions since the way it is now in English it is not clear what you are talking about. Either of your suggestions listed below your translation would be better than that. Give it some more thought and do an edit. If you need help, we will give you some ideas too.
Hi dramati,
I translated that way since the meaning in Portuguese was not clear either. As the person who asked for the translation said that he was talking about "friendly matches" (link below), I will edit accordingly.