Denmark, surprise joint leaders of Group B, won their qualifying section ahead of Portugal, largely due to a 2-1 home victory over them.
"We have closed down Cristiano Ronaldo before and we firmly believe we can do it again," said defender Simon Kjaer, one of the Danes' best performers in a surprise 1-0 win over Netherlands in their opening game at the finals.
"He has got unique individual skills but we have shown before that we can cope with him as a team and we believe that we can defeat Portugal again," Kjaer told reporters.
Midfielder Michael Silberbauer man-marked Ronaldo in Copenhagen and is set to start in place of Simon Poulsen.
Denmark had lost the Euro qualifying away game 3-1 in Lisbon. They were also in the same qualifying group as Portugal for the 2010 World Cup, winning one game and drawing the other.
The Danes, the ninth-ranked team in the world, were initially tipped to go out in the first round of Euro 2012 but 10th-ranked Portugal have lost their way recently and now look vulnerable.
Although Kjaer bullishly said the Danes had "blown the group right open", coach Morten Olsen sounded more cautious in the wake of the 1-0 win over an unusually wasteful Dutch side.
"A bit of euphoria is great but we will keep both feet on the ground. We have to do better against Portugal because they are under a lot of pressure now," he said.
Portugal, who lost 1-0 to Germany, have won just one of their last seven games and scored once in the last four.
"We will try to continue on our path and play well. Let's work to be a little more efficient and look for the luck we've been missing in recent games," said coach Paolo Bento.
Although Bento insisted he was happy with the performance against Germany, he will be under pressure to rework his misfiring forward line.
Helder Postiga made little impact against Germany as the lone striker in Portugal's 4-3-3 formation and must be under threat from young striker Nelson Oliveira, who Bento has tipped as a player for the future.
Portugal only looked dangerous after Silvestre Varela replaced midfielder Raul Meireles and pushed further up the right wing. In his 10 minutes on the field Varela hit the bar with a cross and forced an excellent save from Manuel Neuer.
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