Ronaldo had a dismal performance in Brazil's 1-0 win over Croatia and the team admitted his physical condition was a factor.
Ronaldo arrived for Brazil's training camp overweight, but doctors had said he would be ready for the start.
Ronaldo, along with captain Cafu, underwent a 40-minute muscle training session Friday morning in Koenigstein, just outside Frankfurt.
The striker had been taken to a Frankfurt hospital on Wednesday complaining of dizziness, but results showed nothing wrong and he was given the go-ahead to start against Australia in Munich, according to the AP.
The strike was defensive in nature and came in response to three attacks on the US military in February