The two powerhouses sit on four points apiece but Iran linger just behind on three points, meaning a historic last-16 spot at World Cup 2018 is also within their grasp.
They can take solace from their own performance against a slightly fortuitous Spain and also Portugal’s insipid display to grind out a 1-0 win over pointless Morocco.
With that in mind, we examine some of the key tactical talking points as Team Melli face Seleccao das Quinas at Mordovia Arena in Saransk.
CAN RONALDO BE STOPPED?
Cristiano Ronaldo has been a picture of focus at this tournament, a testament to his developing fitness through the season.
The 33-year-old appears to be peaking in Russia while other big names arrived in brittle condition from punishing campaigns. Indeed, Ronaldo is tearing through teams and attempting to stop him is akin to dancing with a tornado – opponents are being left in ruin.
For Iran to smother his influence, it will require a combination of concentration, coordination and cohesion. And it starts with plugging the supply chain, which while it may seem rather prosaic actually carries the greatest gravitas.
Iran’s five-man midfield has to be perfectly positioned to close off the penetrating passing lanes because if the ball finds Ronaldo – who smartly sits furthest up the pitch when Portugal don’t have possession then drops for the outball – he’s difficult to stop in transition.
Having morphed into a more traditional No9, he now plays the width of the box so concentration will be key to ensure he’s tightly marked.
It’s a huge challenge for Team Melli boss Carlos Queiroz but as one of the better defensive tacticians, you can be sure his team will be supremely organised. They need to be for the full 90 minutes against Ronaldo, though.
TO THE AZMOUN AND BACK
One of Iran’s under-utilised weapons in this tournament is the aerial prowess of 23-year-old frontman Sardar Azmoun.
The Rubin Kazan striker won nine headers against Spain and seven against Morocco – both were game highs – and they are not aimless flick-ons either because Azmoun, at a modest 6ft, has superb spring.
But against Spain in particular, Iran struggled to get team-mates close enough to him to prosper from his intelligent knock-downs.
Given Queiroz sets up his side to abandon thoughts of winning the ball in the opposition half and instead press in packs once they bolt back into their two defensive blocks, it means Azmoun is the first receiver out of the back.
Left on his own, the ability to win the initial battle is going to waste and if he can dominate a frangible Portugal pairing of Pepe and Jose Fonte, Iran have to get a team-mate in his vicinity to build their attacks.
PORTUGAL’S 4-4-2 BECOMING A 4-3-3 IN ATTACK
In their opening two encounters, Portugal have been comfortable in ceding possession to their more enterprising opponents with the aim of hitting back on the counter.
But on Monday, they will be expected to take the initiative as Iran set-up in a compact 4-5-1 and won’t deviate away from their defensive mindset with the pressure on Portugal.
That could result in a change of shape when Fernando Santos’ side attack. The European champions are glued to their 4-4-2 but with a dominance of possession, they could transition into a 4-3-3 with Bernardo Silva pushing further up the right to join Ronaldo and Goncalo Guedes or Andre Silva in the front three.
The impressive William Carvalho is the fire blanket in front of the defence with Bruno Fernandes tucking in alongside Joao Moutinho to feed an explosive attack. Doing this, will help Portugal push necessary numbers in between the lines with the width coming from their full-backs.
Ahmed Musa achieved iconic status on Friday as he joined celebrated Cameroon star Roger Milla as the only two African players to score twice in two separate World Cup matches following Nigeria‘s the 2-0 victory against Iceland.
His brilliant brace kick-started the Super Eagles’ World Cup campaign in Russia, which is ironic as he couldn’t get a kick for club side Leicester City last season – having made a £16m switch to the King Power Stadium from CSKA Moscow in 2016.
The only other African footballer to have scored twice in two separate World Cup matches was Cameroon’s then 38-year-old Milla, both at Italia 90.
Musa spent the second half of 2017/18 on loan back at CSKA, scoring six league goals in 10 outings.
Buoyed by that confidence he scored a terrific opening goal against Iceland, lashing in Victor Moses’ cross, before coolly rounding the goalkeeper for his second.
The result breathed life into Nigeria’s hopes of progression from Group D following an opening defeat to Croatia. They are now second, above both Iceland and Argentina, with the result and display from Musa possibly breathing life into his club career too.
“Let me ask you a question,” he began after becoming Nigeria’s record finals scorer. “Did you ever see me play? Did they give me any opportunity to show myself after they sacked the coach [Claudio Ranieri, the manager who signed him in 2016]?
“So I don’t want to talk about that. I have to go back to the team and finally I say what … maybe I have to leave.
“They’ve (CSKA Moscow) given me the confidence when I came back and made me believe I can do it because it’s not that easy staying on the bench without playing for seven months.”
Nigeria face Lionel Messi and Argentina in their final group clash on Tuesday. They have lost all three of their previous World Cup meetings with La Albiceleste but did come from two goals down to beat the South Americans 4-2 in a friendly in Krasnodar last November.
Defender Leon Balogun, who signed for Brighton & Hove Albion from Mainz before setting off for this World Cup, acknowledged that Argentina would be a wounded animal after their 3-0 hammering at the hands of Croatia.
“I think they haven’t found their form yet and they haven’t found Messi as well,” said the 28-year-old.
“They’re very dependent. They always have been. Again if you look at their qualifying campaign you can see that Messi was the one who took them to the World Cup.”
The lesson from watching Argentina’s defeat against Croatia is patience.
“You have to be very patient and definitely you need to be disciplined defensively,” he said.
“You have to close all the passes, or as many as possible, to Messi to leave him out of the game – but then at the same time take out the rest of their amazing qualities so it’s not going to be an easy one.”
Mexico have been one of the pleasant surprises of the World Cup, having won both of their games so far to top Group F.
After shocking Germany 1-0 in their opener, Mexico topped South Korea 2-1 on Saturday to remain flawless.
Check out the fan reaction of Mexico supporters in Rostov after their team came through again.