You have to remember that he will face Del Potro in the last day of the group stages, night match. Hours later he will be playing his semifinal vs probably Nole or Murray. Is it really wise to go all out against Del Potro, Murray and Nole on three consecutive days given Federer's age? We all know what happened the last time he got into a slugfest with JMDP right before facing Murray, and this time he doesn't even have a rest day.
If he beats Ferrer 2-0, he will be qualified and would definitly be wise not to go all out against JMDP. Going through JMDP, Nole and Murray on three consecutive is always a tough ask, with the diminished recovery powers of Federer's aging body probably an impossible one.
Well, beating Del Potro will probably mean eliminating him, so he won't have to worry about seeing him again in the final. He wouldn't mind seeing Ferrer a second time.
There's also the possibility of Tipsarevic beating Ferrer, in which case a loss to DelPo will give away the group win.