Ronaldo made history as the first player to score in four European Championships with a double in a thrilling 3-3 draw with Hungary. The result saw Portugal just squeeze through behind Iceland and Hungary in third-place in Group F.
The Real Madrid star has more records in his sights as he is just one goal behind French legend Michel Platini's mark of nine in European Championship finals.
However, Ronaldo refused to accept Portugal were underdogs against a Croatia side that shocked two-time defending champions Spain to top Group D.
"We made it and now we face a very good team, but the odds are 50/50," said Ronaldo.
"Croatia are a tough team, with good players. Not every team manages to beat Spain.
"We respect them, but we know our strengths and we'll look them in the eye when we face them."
Croatia are likely to be even stronger for Saturday's clash in Lille (1900GMT) than against Spain with the return of Ronaldo's Real Madrid teammate Luka Modric from injury.
Juventus frontman Mario Mandzukic is also likely to shake off a knock to retake his place after Croatia coach Ante Cacic made five changes for the historic win over Spain.
"Croatia are one of the sharks we had been trying to avoid," admitted Portugal boss Santos.
"They placed first in a group with Spain, which tells you all you need to know about them."
However, Santos has placed his faith in Ronaldo to again answer his country's call after his double strike.
"This will have increased his confidence levels. He's a confident player anyway, but this will have boosted him for the next game against Croatia."