Ronaldo has been confirmed in the starting lineup by coach Carlos Alberto Parreira despite his lethargic performance in the 1-0 win over Croatia on Tuesday.
On Wednesday he was taken to a clinic after complaining of dizziness but Brazil said no abnormality was found.
Australia have been given a clean bill of health after Monday's 3-1 win over Japan but coach Guus Hiddink has suggested he may rest four players who picked up yellow cards in that match, Tim Cahill, John Aloisi, Craig Moore and Vince Grella.
Brazil are unbeaten for 11 internationals and have not conceded a goal for five. Their last defeat was against Mexico at last year's Confederations Cup.
Australia have been on a high since their dramatic qualifying victory over Uruguay last year that propelled them into the World Cup finals for the first time in 32 years.
They beat European champions Greece and Liechtenstein in warm-ups then held Netherlands to a draw before a stunning come-from-behind victory over Japan in their World Cup opener.
Trailing 1-0 after a disputed first-half goal, the Socceroos scored three times in the last eight minutes with Tim Cahill scoring twice and John Aloisi once.
Brazil coach Carlos Alberto Parreira: "Ronaldo will certainly get into form, which is what we want to see.
"Australia are a very physical team but we are not going to get into a fight with them. Our business is to play football, put the ball on the ground and impose our style and technique."
Australia midfielder Tim Cahill: "On Sunday, (it will be) 11 superstars against 11 honest Australian lads who are really looking forward to playing. We have got a lot of respect for them but on the pitch it's going to be a different story."
"All we need to do against Brazil is not concede and we have got every chance of beating them."
Brazil are the only country to have played at all 18 World Cup tournaments and are defending the title they won in Japan and South Korea four years ago.
Their last defeat at the World Cup was a 3-0 loss against France in the 1998 final.
Australia boss Guus Hiddink is the only coach to take two different nations to the World Cup semi-finals, having achieved the feat with Netherlands in 1998 and South Korea in 2002.
