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.