More than 150 spectators were ejected from the tournament grounds after rival fans battled with flag sticks, bottles and chairs outside Rod Laver Arena, the Australian reported.
Some of the hooligans, some chanting "Die Croatians, Die," were wrapped in national flags as they fought on lawns at the Melbourne Park venue.
No injuries or arrests were reported and tournament officials feared that the same hooligans would return Monday to cause more trouble.
Martin Cilic of Croatia was scheduled to play Serbia's Ilia Bozolijac.
Ray Liotta sues skin care company over use of likeness
Aaron Carter is still in love with Hilary Duff