Lindsey Vonn isn't just a huge fan of Roger Federer, she says he's one of her personal heroes ... and praised him as the greatest tennis player of all time!

The ski legend/smokeshow was leaving S.I.'s Fashionable 50 event in L.A. when she told us why Club Fed's the best to ever do it ... over guys like Sampras, Agassi, Nadal and Djokovic.

Might have something to do with the fact RF's just racked up his 19th grand slam singles title despite all sorts of injuries.

Guessing Fed's drive to bounce back inspires Vonn who was banged up pretty badly herself recently ... but continues to beast out in rehab.