Cristiano Ronaldo gets the opportunity to upstage fellow superstar Lionel Messi when Portugal open their World Cup campaign on Monday with a mouth-watering clash against fellow European heavyweights Germany.

Having dethroned Messi as World Footballer of the Year, Ronaldo, who has been battling a left knee injury, declared himself 100 per cent fit to play in the Group G match in Salvador, comfortably the stand-out fixture of the day.

"I'd like to be 110 per cent, but I am at 100 per cent and ready to help the national team," said the 29-year-old Real Madrid star.

"Obviously, I would like to play without pain, but in my entire career I have always had injuries and knocks to deal with, but I am ready to play."

Three-time champions Germany beat Portugal 3-1 in the third place play-off at the 2006 World Cup, 3-2 in the 2008 European Championships quarter-finals and 1-0 in the first round of Euro 2012.

Other statistics weigh against Ronaldo as Germany have won all of their World Cup opening matches since 1990.

Germany have a number of injury doubts going into the eagerly-awaited game.

But goalkeeper Manuel Neuer will return to duty after overcoming a strained shoulder and midfield star Bastian Schweinsteiger will play even though he was helicoptered to a hospital on Saturday for a knee scan.

In the group's other game on Monday, 2010 quarter-finalists Ghana face the United States in Natal having knocked out the Americans in the last 16 four years ago.

African champions Nigeria, who have taken just two points from six matches in their last two appearances at the World Cup, face three-time Asian champions Iran in Curitiba in Group F.

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