Ronaldo may have been heavily criticised for his lacklustre performance against Croatia during the week but Australia said they fear him as the player most capable of ruining their World Cup dream.
Ronaldo was substituted against Croatia, then taken to hospital after complaining of feeling dizzy, but was cleared to play in Sunday's Group F match against Australia in Munich.
Socceroos coach Guus Hiddink said on Saturday suggestions that Ronaldo is a spent force in the game were incorrect and his players had to be on their guard against him.
"He is a player who can be apparently off the pace, sleeping sometimes, but on the other hand he can strike in one or two seconds," Hiddink told a news conference.
"I don't care about the newspapers, I just care that he is dangerous."
Ronaldo did not play when Australia beat Brazil 1-0 in the third place playoff at the 2001 Confederations Cup but scored a hat-trick at their previous meeting in the 1997 final, won 6-0 by the South Americans.
"As a striker myself, Ronaldo's been a player that I've looked up to for years. He's been world class," said Australia captain Mark Viduka.
"His career has been up and down like most of us in football but he's a very talented player.
"People are saying he's out of shape but he's still very dangerous and he's still one of the best players in the world."