Portugal and Denmark are tied on 13 points with Tuesday's meeting between the pair in Copenhagen potentially deciding the group. However, both teams must first secure victories Friday when Denmark is at Cyprus.
Portugal, which is top on goal difference, is without Real Madrid pair Pepe and Fabio Coentrao in Porto and has recalled veteran striker Nuno Gomes and winger Ricardo Quaresma for the last two matches of Euro 2012 qualifying.
''We must not fail and we have to win,'' Portugal coach Paulo Bento said. ''We have to maintain the same ambition.''
Norway also has 13 points but has only one game remaining Tuesday against Cyprus in Oslo.
As ever, Portugal will look to Cristiano Ronaldo for attacking inspiration - the Real Madrid forward has scored four goals in qualifying - along with Manchester United's Nani. Porto winger Silvestre Varela has also replaced Zenit St. Petersburg forward Danny, who is absent for personal reasons.
Pepe and Coentrao's absence disrupts a defense that is already without Ricardo Carvalho, who was suspended for one year suspension after walking out of training ahead of Portugal's last match against Cyprus. Chelsea fullback Jose Bosingwa is also out of favor with Bento, who called up Cologne defender Henrique Sereno for the first time.
''We are pleased with the performance and attitude of the players who play in that position so I have nothing further to add to that,'' Bento said.
Despite recent disruptions, Bento has rejuvenated Portugal's qualifying campaign with four straight victories following a draw and loss to start.
Should both Portugal and Denmark win their games on Friday, Portugal would hold the better head-to-head record against the Danes and would only need a draw Tuesday to secure the group.
Iceland, which has four points from seven matches, will be coached for the last time by Olafur Johannesson before his four-year contract expires. Johannesson is without defender Hermann Hreidarsson and striker Heidar Helguson.
Sunderland striker Nicklas Bendtner will be expected to lead Denmark again after netting twice in a 2-0 victory over Norway last month.
The Cypriots have two points and are trying to avoid finishing bottom of the group. They have never beaten Denmark in five attempts.
Norway needs to score goals against Cyprus to have a realistic chance of grabbing a playoff place, and captain Brede Hangeland is suspended.
''We can but try our best,'' Norway coach Egil Olsen said.