The 29-year-old appeared at a pre-match news conference to declare his readiness for the challenge.

He said: "I've been practising for many days and I am feeling well. There has not been a single moment when I have not played without pain in my career, It's just the burden of my work.

"I'm ready, I'm feeling well. I want to play and help the national team. If I'm not fit, I'll be the first to tell the coach.

"If I feel something, and I am sure I won't feel anything during the game, I will not risk my career."

Ronaldo's last competitive match was the Champions League final victory, and he admitted the World Cup is a fresh challenge.

He said: "My career has been fantastic so far and I just want to continue.

"I relish challenges, I want to do my best in whatever competition I'm in. It drives me to succeed at a tournament.

"Of course I want to win this championship but let's take it one step at a time."

Among individual players at this World Cup, perhaps only Ronaldo's long-time rival Argentina's Lionel Messi holds as much sway over a single team.

Portugal's coach Paulo Bento admitted that Ronaldo's participation was vital against the group favourites.

He said: "A player like Ronaldo is important for any team, and that's the case for us.

"It's good that he is back. Group G has a team in Germany who may be considered favorites because of their history and potential, but it is a very balanced group.

"USA and Ghana are well organised and have some good values, but we have the ambition to move forward, grab the most points as we can and to reach the round of 16."

Portugal's keeper Eduardo was more bullish about Ronaldo, saying he's certain he will be in top physical shape.

"Cristiano is training well, he's training hard and we are sure he's going to be 100% to help us and to have an excellent game," Eduardo said.

"He's healthy and he can't wait to start this tournament. He's determined and he wants to play to the limit, like all of us, with an immense will to win and give our best.

"You're talking about the best player in the world. It's like the other good players in the other teams that can make a difference," he added.

"We're ready for anything, but of course with Cristiano it's better for us. Of course we are a group, a lot of good players and nobody wins alone. But of course with Cristiano it's better for us."

The match, a re-run of Portugal's first match of Euro 2012 where Germany ran out 1-0 winners, should be an intriguing one in a World Cup which is already shaping up to be one of the best.