Hewitt won 7 of his first nine matches against Federer. Fed has since won the last 11 in a row.
Agassi won the first 3, Federer won the last 8. Agassi was still playing very well despite his age and was still beating alot of younger players and was probably still in the top 5 to 10, even until a year or 2 before retirement.
http://www.atptennis.com/3/en/players/headtohead/?player1=Federer,+Roger&player2=agassi