I think that Roddick just has luck in getting easy draws, while Federer has the worst luck. In the French Open for example Roddick's quarter was loaded with Americans and only Fabrice Santoro and Chela as clay courters. Federer had Guga in the third round!
Now in Wimby, Roddick didn't have the caliber of opponents that Federer did. Federer had to face Karlovic, Hewitt, and Grosjean. Roddick's last three opponents were Pop, Shalken, and Ancic.