Ronaldo has admitted he made up his mind to leave Manchester United last summer.
The Portugal winger looks set to seal an Â£80million move to Real Madrid after United accepted a world record bid from the Spanish giants.
"After we won the European Cup. I thought there is no more I can achieve here," the Portuguese star told the News of the World.
Ronaldo added: "When you have done all you can, you know it is time for a new challenge. I stayed one more season and it was nice to make it three titles in a row but it was my dream to play for Madrid."
Madrid conducted a long and public pursuit of Ronaldo last year but it proved ultimately unsuccessful as the 24-year-old remained at Old Trafford.
The fact Madrid have not won the Champions League since 2002 - and are currently comfortably only the second best team in Spain behind Barcelona - has led some critics to claim Ronaldo is more interested in money than medals.
But Ronaldo insisted: "Of course, I have not gone for the money. To go down as the greatest. It will take lots of work but that is my target. If you go down as the greatest at Madrid it means you are one of the greatest of all time."
The winger also apologised for his infamous wink which followed Wayne Rooney's sending off in the 2006 World Cup quarter-final between Portugal and England.
"Sometimes you do things because emotions are running high but Wayne is my good friend," Ronaldo said.
"I can't regret it because I think the whole experience made me stronger. But looking back it was not the right thing to do and not something I would do any more."
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