The 42-year-old was thrilled to see his side bounce back from a cruel defeat to Brazil with the most convincing of victories on Wednesday night
A goal from Ivica Olic before the break handed the Croatians the lead before Ivan Perisic and two strikes from Mario Mandzukic sealed the result in the second half.
"We weren't on top of our game in the opening five to 10 minutes and Cameroon created some chances," Kovac told HTV.
"But then we found our rhythm. Olic scored and that opened up the game for us. We could have scored even more goals.
"I can only congratulate the players, though. They won in very difficult conditions. It was hard to sit or stand, let alone run around on the pitch."
Croatia currently sit third in Group A with three points from two games, trailing leaders Brazil and Mexico by one point.
"The match against Mexico will be like a final. We have to get some rest, because a serious opponent awaits. Mexico are strong, but so are we," said Kovac.
"The Mexico players are very patriotic, just like us, and they have a great team spirit. They play some aggressive football with a lot of pressing. They are strong going forward, but also have some weaknesses in defence, like Brazil showed us.