Ronaldo said he had recovered from a left-knee injury that has bothered him recently.
"I would be the first to tell the coach if I were unable to play, I would never put my career at risk even for a World Cup," Ronaldo said as he prepared for Tuesday's match in Group G.
"I'd love to be 110 per cent and but I am 100 per cent and that is enough to help my team," the Real Madrid striker said.
The Portugal forward and FIFA player of the year was seen earlier this week walking away from training with ice bags on his knee. He wore a bandage on the knee during Sunday's practice at the match venue, the Arena Fonte Nova.
"Obviously I wish I did not have any pain, but that is impossible," Ronaldo said. "I don't think there's ever been a day without pain."
Ronaldo played in his team's Champions League final on May 24 despite earlier doubts about his fitness and scored a penalty to complete Real Madrid's 4-1 victory over city rival Atletico.
Both German and Portuguese teams have stressed that Portugal is not just about Ronaldo. But Portugal's hopes of upsetting Germany will hinge on Ronaldo's ability to score.
"The importance of having in the team a player like Cristiano is enormous, but this does not mean we have to be world champions," Portugal coach Paulo Bento said Sunday. "All 23 players have to be like a compact team in order to achieve a good result."
Ronaldo said "one player is not a team.
"I can make a difference but I am here to help the team."
Germany goes into the match with recent history on its side. Germany has won the last three tournament matches against Portugal: a 3-1 win in the match for third place at the 2006 World Cup; a 3-2 win at Euro 2008 and a 1-0 victory in their opener at Euro 2012.
"Obviously history has been positive for Germany. But Real has beaten Bayern Munich (in the Champions League semifinals) and maybe this will be the year to change the history and Monday would be the best day for it to happen," Ronaldo said. "It's a new tournament, new players, our team is good, we have confidence."
That said, the talismanic forward was wary of raising expectations too far.
"We must be humble. It's a difficult start for us," he added. "We are playing one of the favourites and one of the best teams. I personally and the entire team is very motivated and we want to enjoy the tournament."