1900 BST, Friday 15 June, Fisht Olympic Stadium (Sochi) BBC 1
Portugal’s quest to follow up their 2016 European Championship title with a World Cup win begins against a nation that achieved that feat in 2018 and 2010 (and followed it with another Euro win in 2012). That country is neighbour and closest international rival, Spain.
As group favourites, Spain will look to impose themselves on another major tournament and atone for the debacle of Brazil four years ago when they fell at the first hurdle.
Read RealSport's preview for Morocco vs Iran HERE.
However, their plans were thrown into turmoil on Wednesday morning when Coach Julen Lopetegui was removed from his post after clashing with the Spanish FA over his acceptance of the Real Madrid job. It has since been announced that Fernando Hierro will step into the role during the tournament.
Whoever prevails in this clash should top the group, setting up a potential clash with hosts Russia in the Round of 16, although Uruguay might have something to say about that.
Either way, both teams are expected to progress, so victory could help build momentum and confidence as they head deeper into the tournament.
Last Time Out
Portugal 3-0 Algeria [International Friendly]
Goncalo Guedes boosted his chances of starting against Spain in this World Cup opener with a brace against Algeria last week.
Manchester City playmaker Bernardo Silva, who's expected to take a starring role in Russia, assisted Guedes for his first, before the 21-year-old winger, who spent last season on loan at Valencia from Paris Saint-Germain, headed home the third.
Midfielder Bruno Fernandes scored from Cristiano Ronaldo's cross in between Guedes' goals.
Tunisia 0-1 Spain [International Friendly]
In what proved to be Julen Lopetegui's last game in charge for the Spanish, the 2010 World Cup winners were held for most of the game by Tunisia, who face England, Belgium and Panama in Group G.
Despite 70% of possession, La Furia Roja failed to register a shot on target until the 65th minute and struggled to find a breakthrough, requiring a late Iago Aspas goal to seal the victory.
The win kept Spain's unbeaten record intact and extended it to 20 games.
Portugal will start with 35-year-old Pepe likely paired alongside 34-year-old Jose Fonte in the centre of defence.
Up front, Cristiano Ronaldo and AC Milan’s Andre Silva will provide the firepower, while Manchester City youngster Bernardo Silva can be expected to line up alongside William Carvalho and veteran Joao Moutinho in midfield.
With Gelson Martins and Goncalo Guedes to provide speed out wide, Fernando Santos has a wide array of options.
It gives him the option of playing a 4-3-3 formation, but Santos is expected to stick with the 4-4-2 that delivered at the Euros, especially with the emphasis on safety-first against Spain.
It is not clear what formation Spain will start with in Russia now under interim boss Fernando Hierro, but with the preparation and done and less than two days to work with the squad, Hierro could pick the 4-2-3-1 that saw them through qualifying.
However, David de Gea, Gerard Pique, Sergio Ramos, Sergio Busquets, David Silva and Isco are all almost certain to start whoever is at the helm.
Dani Carvajal, moreover, returned to training, but Friday may yet come too soon.
There has been a lot of debate surrounding the inclusion of Diego Costa, but Lopetegui persevered with the striker and this game could suit him, though Celta Vigo's Iago Aspas has also put himself in the frame for a start thanks to his excellent performances for club and country.
Key Battle: Cristiano Ronaldo (Portugal) vs Sergio Ramos (Spain)
They may be teammates but with national pride at stake, there will be no love lost between these two.
Cristiano Ronaldo had a brilliant second-half to the season, finishing with 26 goals in 27 league matches and 15 from 13 in the Champions League.
However, he was upstaged by Gareth Bale in the Champions League final and will be keen to be the leading man once again.
Sergio Ramos is a player for the big occasion and he will be out to frustrate his Real Madrid colleague on Friday. Expect to see physical tussles between the two early on as Ramos tries to exert his authority.
That same approach led to the withdrawal of Mo Salah in the Champions League final against Liverpool - a moment that turned the game in Real Madrid’s favour.
Keeping Spanish eyes on the prize
With Lopetegui gone, the Spanish camp has been shrouded in chaos and uncertainty, with the Spanish RSFF naming Albert Celades as the new boss before eventually settling on Fernando Hierro.
Regardless of who takes over for the World Cup, the challenge is the same; uniting the squad.
A number of players were keen for Lopetegui to stay and made that clear to the RSFF, so Hierro has to ensure the squad are willing to commit to him without hesitancy or distraction.
Keeping them focussed on the task over the next month amid intense media scrutiny is the most important aspect of Hierro's job.
Keeping a sense of continuity will be vital if Spain are to avoid descending into further chaos during the tournament.
Portugal's unchanged heroes
It is hard to see how Portugal have improved significantly since 2016.
They might look slightly stronger in the attacking midfield area, with Martins and Guedes offering a new dynamism to their attack, but their ageing centre-back pairing could prove to be their Achilles heel.
They won none of their groups games in Euro 2016, but still lifted the trophy, so they have shown they have the mental resilience to beat the odds. However, their best central defenders are now in their mid-30s and could struggle against pacier opponents.
However, they had to ride their luck and there will be little room for that in Russia.
Overloading the midfield
Spain and Portugal's contrasting styles of football actually compliment each other rather nicely, with La Furia Roja taking the active role and dominating possession, whilst Santos sets Portugal up in a low block defence in a 4-4-2.
However, with four in the midfield, it's crucial that Spain dominate this area, as this is the foundation of possession for Hierro's side, boasting a midfield stocked full of wonderfully technical and creative players.
Hence, the decision to deploy a 4-2-3-1.
It utilises Sergio Busquets and one of Koke or Thiago Alcantara as defensive stability, with a three of Andres Iniesta, Isco and David Silva all occupying the central space in the final third and drifting inside.
Thus, five men dominate the central areas, with the fullbacks pushing on to provide width and overloading Portugal's midfield to establish their superiority in this area.
Prediction: Portugal 1-2 Spain
Despite Lopetegui's departure, his unbeaten legacy cannot be ignored and whilst they're not the force of yesteryear, they have the squad to compete with absolutely anybody, irrespective of who's in charge.
Portugal started slowly in Euro 2016 and almost stumbled their way to victory and were thumped by Germany in their 2014 World Cup opener, so the onus is on them to avoid another lazy starts.
The unusual and chaotic situation at Spain might spark a response from the players, many of whom will work with the disposed coach again in just a few weeks’ time.
Listen to the RealSport football writers discuss Group B in Kremlins in the Basement: RealSport’s daily World Cup podcast.