Tournaments hosts Russia succumbed to a heavy defeat against Uruguay earlier this week, but there remains genuine hope of a first-ever appearance in the quarter-finals.
Spain have not suffered defeat in 23 matches - since a 2-0 setback to Italy at Euro 2016 - but Julen Lopetegui's exit before this tournament has suggested that the previous form of the 2010 world champions is now irrelevant.
As long as they remain in the competition, La Roja remain a strong contender for the Jules Rimet trophy but Fernando Hierro's team are yet to fully convince and their performances at the back are undermining any positive work in attack.
Some of the blame - back home at least - has been placed on the shoulders of goalkeeper David de Gea, who made a blunder in the 3-3 draw with Portugal and conceded a further two goals against Morocco.
There had been reports which suggested that the Manchester United stopper would be taken out of the firing line on Sunday, but Hierro has revealed at a press conference that De Gea will remain in goal.
Eight of the squad have started in each of the group fixtures - suggesting that Hierro is looking for continuity - but it has been claimed that there will be alterations in midfield.
Whether that will benefit Spain remains to be seen but Hierro is still searching for the right balance, with only a VAR-assisted equaliser against Morocco saving the newly-appointed coach from embarrassment.
What Spain have secured is a place - on paper - on the easier side of the draw. They can reach the semi-finals without playing any of the standout nations in the world with Brazil or Belgium awaiting them in the last four.
No-one - especially Hierro - will look too far ahead given the nature of their last three outings, but the platform is there for Spain to continue to warm into this competition before peaking at the right moment.
Recent form: WDWDWD
While Spain came into this competition having not lost in 20 matches, Russia found themselves at the other end of the spectrum after failing to register a win in seven outings.
However, tournament football - especially in your own country - has the potential to change the mood of a nation and unlike three weeks ago, there is now genuine belief that Russia can progress through to the quarter-finals.
There will also be a sense of perspective with the victories that they have recorded coming against poor Saudi Arabia and Egypt teams, but they were results which put a smile on the faces of their supporters and it has led to many of them being prepared to invest themselves into the emotion which comes with this event.
Russia were outclassed against Uruguay and deservedly lost by a wide margin, but almost an hour was played with 10 men and it is a result which has not been widely criticised by the home press.
Whether that is an attempt at false optimism, only time will tell, but the Russian support and media no longer consider their team as also-rans and Sunday's contest in St Petersburg has the potential to be one of their most famous days.
There will be concerns over a backline that required the recall of 38-year-old Sergei Ignashevich, but going forward, stars are being created with Denis Cheryshev and Artem Dzyuba both impressing having started the tournament on the bench.
Russia will require a strong performance from the influential Aleksandr Golovin - as well as captain and goalkeeper Igor Akinfeev - but they are players who are for the big occasion and it does not get much bigger for Russia at this moment in time.
Spain begin the game as favourites but if Russia start well, set the tone and even find an early goal, there will be the increasing possibility of another host nation having a prolonged run at this competition, with three of the last five having progressed through to the semi-finals.
Recent form: LLDWWL
Kepa Arrizabalaga had been suggested as a possible replacement for De Gea, but Hierro has dismissed those claims and will start with his current number one.
Yuri Zhirkov and Golovin are also likely to feature, having been dropped to the bench for the Uruguay showdown.
Spain possible starting lineup:
De Gea; Carvajal, Pique, Ramos, Alba; Busquets, Koke; Isco, Silva, Iniesta; Costa
Russia possible starting lineup:
Akinfeev; Fernandes, Kutepov, Ignashevich, Zhirkov; Zobnin, Gazinskiy; Samedov, Golovin, Cheryshev; Dzyuba
Head To Head
Sunday's fixture will represent the seventh match between the two teams, and Spain remain unbeaten having put together four victories and two draws.
Half of the previous contests have taken place at the European Championship, with Spain claiming a 1-0 triumph in 2004 and registering two victories at the 2008 edition.
The most recent clash took place in November, with a 3-3 draw being played out in St Petersburg.
We say: Spain 3-1 Russia
Neither side will be completely satisfied with their performances at this event, but we are backing Spain to begin to find their 'A' game now that the group stages have come to a conclusion. Russia will be confident of causing an upset, but Spain's greater quality in attack could prove to be the difference.