Portugal and Spain, who take on already ousted Morocco, are on four points and need a draw from their respective matches. But Iran are a point behind with three and must beat Portugal to progress.
Cristiano Ronaldo has scored Portugal's all-four goals in the World Cup so far and the European Champions will be heavily dependent on the talismanic forward to see them through against a stoic Iran side in their final Group B match on Monday.
Ronaldo's hat-trick in the 3-3 draw against Spain gave Portugal a vital point while an early headed strike, which made him Europe's all-time top international goal scorer, against Morocco proved enough to bring home three points.
Portugal and Spain, who take on already ousted Morocco, are on four points and need a draw from their respective matches. But Iran are a point behind with three and must beat Portugal to progress.
The European sides will not only aim for victory but will also try to outscore each other with the top spot in the group set to be decided on goal difference.
The 33-year-old Real Madrid forward's individual brilliance has glossed over Portugal's dour performances in their first two matches and Fernando Santos's side have mostly appeared like a one-man team.
In 26 matches competitive matches since Santos took over in September 2014, Portugal have won 20 and lost twice, with one of those being a penalty shootout defeat by Chile in the 2017 Confederations Cup. Of those 20 wins, 10 have been by a single goal and most of the others were against weaker opponents.
They lost the ball numerous times against Morocco which left Santos unimpressed.
"We have to look at this, we lost control of the game," Santos said. "We misplaced a lot of passes, we lost confidence... it was inexplicable.
"If in a match against players like they have, if you don't have the ball, they wear you down and you get into trouble."
On Monday, Portugal's players will meet a familiar face in the opposition technical area in Iran coach and compatriot Carlos Queiroz, who coached the side from 2008 to the South Africa World Cup in 2010.
The stability and pragmatism of Queiroz's seven-year reign, which will come to an end after the tournament with the Portuguese deciding to quit, has firmly established Iran as Asia's number one team.
Iran benefited from a late own goal by Morocco to get their World Cup off to a winning start in St. Petersburg but then lost 1-0 to Spain in a hard-fought match in Kazan.
Stopping Ronaldo from scoring will be the team's primary objective, while young Sardar Azmoun will lead the attack hoping to take the side past the group stage for the first time.
Queiroz has declared the Portugal match as the most interesting and important match in his seven years with Iran.
"We go for it, with great belief and great determination to try to win the game," Queiroz said. "When we arrived, no-one gave us a chance, but we still fight for our dreams."
