Round two of the group fixtures in any World Cup is when patterns begin to reveal themselves as nerves settle, and we begin to get an idea of who is going to be successful at the tournament, and who has reason to fear the worst.
2010 winners Spain will be looking to move into the second stage of Group B with a positive result to build on their 3-3 draw with Portugal, as they showed some impressive attacking football but found themselves repeatedly pinned back by a scintillating Cristiano Ronaldo.
Iran, however, surprisingly lead the group after round one, after a shocking win over Morocco, and will be hoping the resolute defending that served them well in their first match will be enough to keep out one of the pre-tournament favourites.
Iran, heading into the tournament, had registered the second best qualifying record in the world in the final stage of the Asian qualifiers. They won nine of ten games, conceding just three goals, two of which came in the last match, by which point they had already secured qualification.
There were concerns about their ability to score goals, managing just ten over the same ten games, but they knew that if they could keep their solid defence in shape then they would have a chance at registering a good result or two in Russia. Iran did just that, holding Morocco until the 95th minute when an Aziz Bouhaddouz own goal put Iran in front, and sparked wild scenes in the Iranian camp.
Spain headed to Russia in similarly impressive form, romping their qualifying group, but while a draw with Switzerland and a limp 1-0 victory over Tunisia did little to dampen spirits. The worst was feared after the Spanish FA got rid of manager Julen Lopetegui just one day before the tournament got underway after he agreed to take over at Real Madrid.
However, the managerial situation didn't seem to bother the players on the pitch, as they played out a World Cup classic with European Champions Portugal; looking impressive, despite some defensive frailties.
World Cup Highlights
For Spain, this one is irrefutable. Their one and only World Cup win came in 2010, as a golden generation of Spanish players entertained the world with their brand of passing football on their way to lifting the trophy against the Netherlands in South Africa.
In Iran's case, some would make the case for their victory over Morocco as an all-time highlight, helped on by the dramatic fashion in which they won the game.
However, the country's greatest World Cup triumph to date came in the only other World Cup finals victory in their history - a 2-1 victory over the United States at France '98.
There isn't much to report in either camp with regards to fresh injuries or suspensions, as both sides have fully fit squads to choose from.
Spain are boosted by the news that first choice right back Dani Carvajal, who has recovered from the injury he sustained in the Champions League final and will again be available for selection.
Iran (4-5-1): Beiranvand; Hajfasi, Cheshmi, Pouraliganji, Rezaeian; Ansarifard, Amiri, Shojaei, Ghoochanejhad; Azmoun
Spain (4-3-3): De Gea; Alba, Pique, Ramos, Carvajal; Iniesta, Busquets, Thiago; Isco, Costa, Silva
Iran will be boosted by their win over Morocco, a win that takes the pressure off somewhat, and means that any distant last 16 qualification hopes do not hinge on getting a result against the 2010 champions.
However, while their defensive rigidity may make them a stronger opponent for Spain than many suspect, it's hard to look past Hierro's side for this one. They'll be determined to record a win at the second time of asking, and shouldn't be challenged too much at the back by an Iran side ill-equipped to take advantage of some of the defensive frailties exposed by Ronaldo and co.
Ultimately, Spain should have too much quality for Carlos Queiroz's side, and if they can call on any of the attacking prowess they showed against Portugal in their opening game, they should be comfortable winners.
Prediction: Iran 0-4 Spain