it's a legend that Ferrer is the king of consistency and never gets tired or injured, actually in last years he's regularly lost matches against lower ranked players because of that : in those matches he made a lot of unforced errors, that's how you can see it mostly.
And he withdrew from Toronto and Cincy last year.
Even though yes, tiredness and injuries happened less to him last year than previous ones, he's vulnerable to them like everyone.
And he withdrew from the exho with Fed because of a bad shoulder, not because he would be that much clever and less interested in money as Nathaliia suggested
sure he definitely is 'tired' sometimes BUT it's not often people notice it because he is a fighter even on bad days. if you compare the # of matches he plays overall with the number that he APPEARS tired in a season, he still seems pretty kick ass in the energy/effort department compared with a majority of the tour