Atletico Madrid striker Diego Costa struck again for Spain to seal a 1-0 win over World Cup Group B opponents Iran, who had a goal disallowed for offside.
Costa, who scored twice for Spain during their 3-3 draw against Portugal on Friday, added his third goal of the tournament when Ramin Rezaeian's clearance cannoned off the Atletico Madrid striker's leg and into the net.
Iran thought they had leveled things up 10 minutes later, but Saeid Ezatolahi's goal was ruled offside by the referee's assistant, a decision then backed up by VAR.
The result puts Spain top of Group B ahead of Portugal - who beat Morocco 1-0 earlier in the day - on the 'fair play' rule, with both having four points and the same goal difference, but Spain receiving fewer bookings.
Iran is a point behind in third and could still make the knockout stages.
The first half lacked clear-cut chances, with Karim Ansarifard firing a speculative effort wide in the 14th minute before David Silva struck a fierce drive five minutes later, but it hammered into the body of Majid Hosseini.
Silva had the best chance of the opening period on the half-hour mark. Spain took a short corner and after Gerard Pique and Sergio Ramos headed the ball back across the face of goal from Andres Iniesta's cross, Silva went for the overhead kick at close range but saw it fly over the crossbar.
The second half started at a great pace, with Alireza Beiranvand forced into two great saves just six minutes after the restart, denying Pique on the line before a great leaping save to keep out a thunderous effort from Sergio Busquets.
Iran went close at the other end not long after, but Ansarifard's strike could only skim the side of the net.
Costa put Spain ahead in the 54th minute, although there was a touch of luck to the goal.
Iniesta slipped the ball through the middle for the Atletico Madrid striker, who turned as he made his shot, which took a deflection off the legs of Ramin Rezaeian and himself before nestling into the back of the net.
Ten minutes later, Iran thought they had nabbed the equaliser, but they were denied by an offside flag.
Rezaeian pumped a free-kick into the box, which found Ezatolahi at the back post and he slotted home past De Gea.
However, as he celebrated wildly with his team-mates, the referee had his arm up to signal he would be reviewing the goal, which after a tense few minutes, was ruled out for offside in the build-up.
Iran continued to have the better of the efforts as the half wore on, with Mehdi Taremi missing two glorious chances.
The first in the 76th minute saw him just miss a cross into the area from Vahid Amiri by mere inches, before he powered a header high over the crossbar from close range with Iran unable to capitalise on their late pressure.
Iran can still mathematically make it through to the knockout stages, and take on Portugal in their final group game on Monday 25th June, with Spain also playing Morocco on the same in Kaliningrad.
Both games kick off at 7pm.