The Kazan Arena is set to host would could prove to be a decisive match in Group B.
Iran head into this clash with Spain as the surprise leaders of the group after narrowly beating Morocco by a single goal to nil in the opening round of games.
A heartbreaking 95th-minute own goal from Morocco’s Aziz Bouhaddouz handed Iran their first win in a World Cup finals since 1998, blowing the table wide open.
Spain, meanwhile, were held to a draw by a Cristiano Ronaldo-inspired Portugal in arguably the most entertaining game of the tournament so far.
Managerial chaos in La Roja’s camp before the tournament started threw their preparations up in the air, but anything but a win on Wednesday will give leave the European giants’ chances of progressing hanging by a thread.
1 Can Iran upset the odds once more?
Few gave Iran and Morocco much of a chance of emerging from Group B before the tournament kicked off but Day 1 results might have ripped the script up.
Though Iran were rather fortunate to emerge from the first match with all three points, it was a win that the team would have emphasised as the more important of the three.
With Portugal waiting in the wings for Iran's final match and, with the Spanish up next, Iran needed to beat Morocco to boost their chances of progression.
Despite being outplayed and less creative than their first game opponents, the Iranians will take great confidence into another game in which they'll see much less of the ball.
Their resilience and ability to defend and stay in games will go a long way against the Spanish, who may have to be patient.
2 Spain to hit back?
If not for the brilliance and determination of a certain Cristiano Ronaldo, Spain and Iran may well have been facing off joint top of Group B.
Having twice gone behind to a controversial penalty and uncharacteristic David de Gea mistake, Spain showed spirit to fight back and eventually take the lead through a superb Nacho strike.
The Spaniards looked on course to take all three points and put their in-house problems to one side before Ronaldo inevitably stole the show with an 88th-minute free-kick.
The result would have angered Spain, who will feel as though they should be heading to Kazan with three points on the board. Are Iran on a hiding to nothing, then?
This game is a must-win for the Spanish if they are to avoid facing an embarrassing early exit.
3 Changes for La Roja?
Manager Fernando Hierro picked arguably Spain's strongest, most technically able side for the first game but now changes could be in the stand-in boss' plans.
Thiago Alcantara may replace Andres Iniesta in the midfield with the Barcelona man being substituted after 70 minutes in the first game.
Lucas Vasquez, Iago Aspas and Marco Asensio will all be pushing to feature in some capacity, with the latter not being used at all in the 3-3 draw on Friday.
The headline act of Ronaldo stole recognition that his Real Madrid teammate Isco probably deserved. The attacking midfielder was a creative and attacking threat throughout and will be a problem for the Iranian defence.
4 What might Group B look like after Wednesday's games?
With Portugal and Morocco playing in the earlier kick-off on Wednesday, the picture and dynamic of the group may completely change before this game gets underway.
A win for Portugal over Morocco will heap the pressure on Hierro's men, and anything but a win in reply in those circumstances might see the Spanish heading home much earlier than planned.
Should the script be stuck to, wins for Portugal and Spain would see them sit joint-top of Group B before the final game, eliminating Morocco completely.
If that were to be the case, a defeat for Iran isn't the end of the road, though it would leave them needing to beat Portugal in their last game to have any chance of going through.
A victory for Iran, though, would see them progress to the Round of 16 in the tournament, regardless of the result in the other fixture.
5 Carlos Queiroz's hidden secrets?
Group B's draw spat out a demanding task for both Morocco and Iran, although manager Carlos Queiroz will take positives from his links to Spain and his native Portugal.
Having managed his national side on two occasions, most recently right before starting in his current post, Iran will hope his inside knowledge and connections can benefit them.
Queiroz has also managed in Spain, taking charge of giants Real Madrid in the 2003/04 season. His knowledge of the game in south-west Europe is deep, and while it may not be optimally current, tactics on how to counter-act the style of play could come to the fore.
Spain will dominate possession and will probably be the team to create more chances over the 90 minutes though Iran's demonstrated ability to soak up pressure and try to counter-attack could come in handy once more.
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