To do so, on his 136th appearance for his country, the Real Madrid goalkeeper will have to thwart his club team-mate and Portugal captain Cristiano Ronaldo, who has three goals at the tournament and has been on scintillating form.
Eliminating the Portuguese, who have only beaten their Iberian neighbours once in seven competitive games, would give Spain a chance to become the first nation to win back-to-back continental titles with a World Cup in between.
Casillas believes former World Player of the Year Ronaldo has still not quite reproduced the form that helped Real win La Liga for the first time in four years last season.
"His season at Madrid was incredible in terms of the way he played, the goals and the records," the 31-year-old, known in Spain as "Saint Iker" thanks to his miraculous saves, was quoted as saying in Monday's Marca sports daily.
"I don't think that right now he is at his best level," he added.
A fired-up Ronaldo has been peppering opposition goals with shots in Portugal's four Euro 2012 outings and Casillas said he will have to be wary of any deviation in the air due to the peculiar way Ronaldo strikes the ball.
"The ball does something strange," he told Marca, adding that those being used at Euro 2012 did not bend and swerve as much as the "Jabulani" used at the World Cup two years ago.
"It doesn't quite reach those extremes but if you give it a good whack it always swerves in a deceptive way," he said. "I'll have to be on my guard."
Spain's last competitive meeting with the Portuguese came at the 2010 World Cup, when they won a close encounter 1-0 thanks to a second-half David Villa strike.
Spain struggled to penetrate Portugal's defence in that match in Cape Town but the introduction of towering striker Fernando Llorente early in the second half changed the game.
Llorente, whose strength and height were too much for the Portuguese centre backs, could be an option on Wednesday if the game follows a similar pattern, defender Gerard Pique said.
"He used his power in the air to great effect and fought like a lion," Pique told Reuters in an interview at Spain's training base in Gniewino, northern Poland on Monday.
"He could be an option for Wednesday and he is training very, very well," added the 25-year-old.
"He is one of the players who has yet to feature but for sure he will get his chance and that could be against Portugal."
Pique knows Ronaldo and fellow winger Nani from his time at Manchester United and said it would be wrong for Spain to think Portugal were a one-man team.
"Nani is a player with a lot of quality in one-on-one situations," he said. "He can pop up on the right or the left and is a born goal-scorer with a powerful shot.
"We will have to use all the weapons in our arsenal to stop him and Ronaldo."
The winners of Wednesday's game will meet Germany or Italy in Sunday's final in Kiev.
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