The Al Nassr striker was highly critical of himself and his team's performance in the first leg in Copenhagen, where Portugal lost to a Rasmus Hojlund goal, but he is confident the home crowd can play a key role in turning the tie around.
"The air is more tense. I'm not hiding it. We're in a tense moment because we need to win, but that's the beautiful thing about football," Ronaldo told reporters on Saturday.
"Tomorrow, I'm calling on the fans to be with us. Let them give us their strength, because we're going to try to do our best.
"I've lost games in 90 minutes, but I've never lost in the first leg. There are games like that, there are bad days. I didn't play at all the other day, the team didn't play at all, but it's part of life.
"Tomorrow I want to leave the Alvalade Stadium with my head held high. If I score, I'll be happy, but if I don't score, let someone else score, what I want is for Portugal to win."
Ronaldo denied any lack of attitude in the team and was confident they could unite and progress to the semi-finals.
"There's always attitude in the national team. Technical aspects, of course, but other things were lacking too. That's football, you can't always play well," he said.
"The atmosphere wasn't favourable either. I've played 50,000 bad games, and so has the team. Our players, who are used to playing on big stages, have also played bad games. That's part of it.
"There's no need to be nervous. The past is the past. I know there are a lot of people who want us to lose, but if the few who are there are united and have good energy, I'm sure we'll get a great result tomorrow."
Ronaldo also defended coach Roberto Martinez who has been criticised by some pundits and fans.
"I think it's unfair to criticise the coach because we're all in the same boat," the 40-year-old said.
"We lost the game and played badly, but we have the second leg tomorrow. Calm down. Think positive and think that things will go well."