Hewitt defeated Federer where both of them were young, and Federer wasn't formed as player. In the last 14 meetings since 2003 the record is 13-1 for Federer. In 8 times they meet in GS Federer won all the games. So, really Hewitt record against Federer is catastrophic after Federer was formed as a player.
About Sampras and Agassi, in that time there weren't winning that much GS, both were in their thirtieth when Hewitt defeated them so that's not that much. Don't get me wrong, Hewitt was a great player and I really liked him but he was not even near greats of this century like Federer, Nadal or even Djokovic. About Rodick he was even more far of their level.