Kovac's men arrived in Brazil on Tuesday ahead of finalising preparations for their Group A matches against the hosts, Cameroon and Mexico.
Mexico have claimed pre-World Cup friendly wins over Israel and Ecuador, while Cameroon held heavyweights Germany to a draw.
Kovac said those matches were an indicator as to how tough it would be for his team, even with Mexico losing midfielder Luis Montes to injury and having defender Rafael Marquez in doubt.
"The first few days we'll train a little lighter, with final preparations for the game against Brazil after the friendly against Australia (on Friday)," he was quoted as saying by the Croatian Football Federation.
"Our group is tough with Cameroon – who proved themselves against Germany – and Mexico, who I watched and saw their quality.
"Mexico have lost two first-team players to injury, but they remain a tough opponent."
