Off topic, I know, but not to beat a dead horse, Forbes had Federer at no. 31 on their most powerful celebrity list. If I recall correctly, that's the highest tennis player on the list, ahead of Nadal, Sharapova, and Serena Williams; Djoker didn't even make the list. It was based on exposure I think, as in television, print, radio and online?
http://www.forbes.com/sites/dorothypomerantz/2012/05/16/celebrity-100-how-we-create-the-list/
Logically, if you're not marketable, you wouldn't have exposure, and further, Fed's endorsement portfolio is more diverse, and larger than anybody in tennis. If you're not marketable, than companies wouldn't sign you.