Ramos said he preferred to be playing alongside Ronaldo rather than against the star forward, but he is simply hoping the 33-year-old is not at his best at the Fisht Olympic Stadium.
"I'd rather have him on my team than against me. We know how dangerous he can be," the Spain captain told a news conference.
"He ended the season in outstanding shape. He's a constant threat.
"Not only Ronaldo but the Portugal squad are very talented. There are many players who can hurt you quickly, a lot of skill and safe in their defensive formation.
"We will see a match for all sports lovers. It will be beautiful. I hope Cristiano doesn't play his best match."
Spain go into the opener after a tumultuous week during which head coach Julen Lopetegui was sacked and replaced by Fernando Hierro.