Hewitt had beef with lots of the Argentine players. Chela spat in his face, some other guy kept grabbing his junk (the Argentinian's, not Hewitt's) during a davis cup match and so on... don't know where it all started. probably one "C'MON!!" too many or something.
Chela Spit:
http://www.youtube.com/watch?v=0rf7aSEtsTY
Davis Cup junk grabbing:
http://youtu.be/PFKjT5D3F5w?t=59s
Not the best ambassadors for the sport, these guys...