Well - if Ferrero's adductor/abductor (which one was it?) isn't bothering him, and he isn't tuckered out from playing at the hottest time of the day for the 800th match in row (god bless those aussie organizers), then I don't think this should be a problem for him.
It's going to be an interesting matchup. Mario finally seems to have found that form that helped him blow past Rogi at Wimbledon last year -- but Juan Carlos is also playing the best I've ever seen him play. Even though I like both players very much, I'll be pulling for Juanqui. Don't be surprised if it goes 5.
In seeing recent matches (i.e. last few years), there is nothing too flashy about his play (it can get boring and "ugly," really) and there is nothing flashy about his on-court personality either. So why is he so popular? Are people living in memories of long hair and neon clothing??