With Spain unbeaten in 20 games and Portugal having lost one competitive match since September 2014, Friday's clash in Sochi appeared to be a case of an irresistible force meeting an immovable object -- at least until Wednesday.
Then, in a drastic and sensational move, Spain fired coach Julen Lopetegui and replaced him with Fernando Hierro, whose only previous senior coaching experience was at second-tier Spanishc club Oviedo.
Lopetegui, who only last month extended his contract as Spain coach, agreed on Tuesday to take over at Real Madrid next season -- but the Spanish federation (RFEF) only found out five minutes before the official announcement and were furious.
"I am very sad, but we have a magnificent team and hopefully we'll win the World Cup," Lopetegui told reporters on his departure from Spain's training base in Krasnodar on Wednesday.
The contrast with Portugal and the manner in which their coach Fernando Santos agreed to renew his contract after winning Euro 2016 two years ago could not have been greater.
"We sat down, had a coffee and easily reached an agreement," he said at the time.
The neighbourly rivalry and the intriguing prospect of a duel between Real Madrid team mates Sergio Ramos of Spain and Portugal's Cristiano Ronaldo meant the Group B match was always going to be one of the highlights of the opening round.
The Spanish turmoil has added even more spice to the fixture and it remains to be seen how much the Spaniards will be affected, especially after media reports that the squad was split over whether Lopetegui should remain or not.
Some teams can be galvanised by such episodes, however, and Ramos quickly issued a war cry on Twitter.
"We are the national team, we represent a badge, colours, a fanbase and a country," he said. "Our responsibility and commitment is with you as well as for you. Yesterday, today and tomorrow together."
Spain had certainly rediscovered their touch under Lopetegui, using relentless possession to wear down opponents, as shown by their unbeaten run since he took the reins.
European champions Portugal have lost only once in 24 competitive matches since Santos arrived in September 2014 although, despite the presence of Ronaldo and players such as Bernardo Silva, they tend to be dour and pragmatic.
Spain have dominated matches between the pair, winning 16 of their 35 meetings with only six wins for Portugal. Four of those have been at major tournaments with Spain winning the last two - at the 2010 World Cup and Euro 2012.
With Morocco and Iran making up the group, both Iberian teams could be forgiven for being happy with a draw when they clash on Friday but Santos has other ideas.
"I am not signing off on anything other than a win," he said. "We've come here to win every game that we play."