Brazil striker Ronaldo is confident of lining up against Australia in Munich on Sunday despite having to undergo hospital tests on Wednesday.

The 29-year-old was taken to a clinic after suffering dizzy spells and headaches, but doctors could find nothing serious wrong with him.

"I felt quite sick during the day off (Wednesday) and at about three o'clock in the afternoon the doctor decided to take me to hospital just to make sure there was no serious illness or anything," said the Real Madrid star.

"I went to hospital and did a series of tests. They didn't find anything abnormal.

"Today I feel fine. I feel fairly relaxed. It's nothing serious. I've been treated. Today I had a session in the gymnasium and I'm just thinking about the game against Australia."

While Ronaldo is happy with his health, he is less than satisfied with his form, after a poor performance in the holders' opening World Cup game against Croatia led to him being substituted.

He added: "To be sincere I wasn't happy in my performance against Croatia, but the most important thing was that we won. In the next game I hope to do things a bit better."