Ronaldo, 41, was reportedly rushed to a hospital Friday night.
The former Barcelona and Real Madrid star was spending a few days on holiday on the island of Ibiza when he went to the hospital at around 18:30 in the afternoon because he was suffering from chest pains.
He tweeted: "Friends, I had a strong flu picture in Ibiza and I had to be boarded on Friday but it's all in order".
"Tomorrow I will be discharged and I will return home".
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Ibizan newspaper Diario de Ibiza said he had requested to be transferred to a private hospital, Clinica Nuestra Senora del Rosario, later the same day.
Hospital sources have said he is still in intensive care but making good progress.
Ronaldo, who was named FIFA's world player of the year three times prior to his retirement from football in 2011, is a regular visitor to Ibiza, where he owns a house.
Throughout a prosperous career, he scored 62 goals in 98 worldwide appearances for Brazil.
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The former Real Madrid forward, who also played for Barcelona, Inter Milan and AC Milan, thanked his fans for their love and well wishes.
He hit eight goals and scored in every match in Brazil's 2002 World Cup win apart from the quarter-finals en route to lifting the trophy in Japan.
He picked up the Ballon d'Or award for the best player in the world in 1997 and 2002.
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