Brazil's Ronaldo insists he is fit to play against Australia on Sunday.
Ronaldo was strongly criticised for his performance against Croatia
The Real Madrid striker visited a clinic on Wednesday complaining of dizziness and headaches.
But he said: "I felt quite sick on Wednesday and the doctor decided to take me to hospital and did a series of tests, but nothing was abnormal.
"Today (Thursday) I feel fine and relaxed. It's nothing serious. I had a session in the gym and I'm thinking about the game against Australia."
Ronaldo was criticised after a lack-lustre performance against Croatia in their opening 1-0 win and was substituted.
But Brazil coach Carlos Alberto Parreira has said he will start with Ronaldo in the team's next match against the Socceroos at the weekend.
The 29-year-old was then admitted to the clinic on Wednesday where he was given the all clear to train at the gym on Thursday.
In the next game I hope to do things a bit better
And Ronaldo is hoping to put the poor performance of Tuesday night behind him.
He added: "To be sincere, I wasn't happy with 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."
Ronaldo has been hindered by a series of nagging injuries this season.
His last game for Madrid was on 8 April, when he limped off after one hour with an injury to his right thigh.
He fell victim to a mystery ilness hours before Brazil's 3-0 World Cup final defeat by France in 1998 but the Brazilian football association (CBF) said the that problem was not related to the current illness.