Spain will be looking for their unlikely new World Cup figurehead Diego Costa to strike again against already eliminated Morocco on Monday as they bid to outscore Portugal and grab top spot in Group B.
A draw in Kaliningrad would guarantee the 2010 winners safe passage into the last 16 but to be assured of winning the group they need to win a scoring contest with Portugal, who play Iran at the same time.
If the two European sides win by the same scoreline, top spot will be determined by who has the cleaner disciplinary record, with Spain currently on one yellow card to Portugal's two.
Iran, however, would reach the last 16 if they sprang a surprise by beating the European champions.
Costa has scored three of Spain's four goals in Russia, emerging as a surprise talisman after the sacking of coach Julen Lopetegui threatened to throw their campaign into chaos.
Brazilian-born Costa had flopped at the 2014 World Cup after switching loyalties to Spain, failing to have a single shot in two games as the holders were dumped out.
He was left out of the Euro 2016 squad and his return had been a constant subject of debate in Spain, with leading newspaper El Pais questioning his suitability in a team which prioritises possession.
Costa got off to a flying start in the opening game against Portugal, scoring the first equaliser in the 3-3 epic with an emphatic demonstration of his power and skill before also showing bravery to dive in and score the second.
Against the combative and defensively dogged Iran, he broke the deadlock with the scrappiest of goals to give Spain a vital victory.
"He has understood that here he doesn't have to touch the ball very much and he has focused on shooting," said Spain midielder Koke, praising his Atletico Madrid teammate for finally finding his role in the Spain team.
"Instead he stands out for his fighting spirit. He is most comfortable when he is battling with defenders. That's his speciality."
Costa will be on the lookout for more goals against Morocco, who are heading home after losing 1-0 to both Iran and Portugal.
Still, though, it is unclear if winning the group would hand them an easier game in the next round. If Spain qualify, they will meet either rampant hosts Russia or Uruguay, who will battle to top Group A.