Ronaldo's 95th-minute header gave Portugal a dramatic 1-0 win in Copenhagen in Group I of Euro 2016 qualifying.
The result helped Portugal respond to a disappointing 1-0 home defeat to Albania last month and get their qualifying campaign back on track.
Michael Krohn-Dehli hit the post in the first half for the hosts, but Portugal had more chances throughout the contest.
And Ronaldo, who was not at his best - perhaps due to a knee niggle - finally converted one when he nodded Ricardo Quaresma's cross at the death.
Afterwards, Santos said: "Cristiano is Cristiano. He is a winner, and there are no adjectives to describe him."
"We knew that Denmark were very strong.
"I think they saw the match against France and changed from their normal tactics, from a 4-3-3 to 4-4-2 trying to control the match."
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