The teams played to a draw in what was likely the best group-round game we'll see in the tournament.
Spain made a deserved breakthrough in the 54th minute through Diego Costa and earned the much-needed lead. The powerful striker turned in the area and took a shot that deflected off Ramin Rezeian before bouncing back onto Costa's knee and into the net.
Reaching the Round of 16 at the expense of either Spain or Portugal could just be one for the ages.
Hierro said he expects a hard game for every team.
Spain scraped a lucky 1-0 win over impressive Iran thanks to a deflected Diego Costa goal, as the equaliser was ruled offside with VAR.
Iran remain on three points and can reach the knockout rounds for the first time with victory over Portugal.
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Meanwhile, Spain opened the 2018 World Cup with an exciting game against Portugal last Friday.
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Iran's former Real Madrid manager Carlos Queiroz admits his side have not been given a chance against the 2010 world champions.
Iran approached the match with a defensive gameplan in mind.
"We will draw conclusions and continue playing as a team".
Labelled the "Iranian Messi" Azmoun will be facing Spain in the familiar surroundings of the Kazan Arena. Iran, meanwhile, face Portugal on the same day needing a win to progress at the expense of Cristiano Ronaldo and Co.
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Iran responded six minutes later and were unlucky not to have leveled scores.
First place in the group could be decided by goal difference.
Diego Costa was the match victor for Fernando Hierro's side in the encounter after they were held to an enthralling 3-3 draw last Friday by Portugal.
Win or lose, Spain's outspoken central defenders aren't wary of controversy.
Iran was risky from set pieces and its restless defensive display was impressive. And a few minutes before kickoff, there were scenes of solidarity between the starters as Sergio Ramos gave Andres Iniesta a kiss on the cheek and held his teammate's head in the tunnel leading to the pitch.
The expectation that they would play well back was unfounded, however, even if they kept their shape superbly, restricting Spain to a handful of chances in the first half. It appeared that Beiranvand exaggerated his pain after Costa stood on his foot.
"What game were you watching?" said Costa when probed by a Spanish journalist. "They were provoking us, and we were on the grass all the time". This was a unique opportunity for my players to learn about global football.
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