"This is a must-win game for Nigeria, and that makes it a little bit psychologically different - they have to win", Iceland coach Heimer Hallgrímsson said.
Brazil got their eye in after the break, though, finally fashioning the clear-cut opportunities they had been threatening.
Brazil clinched three crucial points, however, with two stoppage-time strikes - and Firmino was crucial in the opener, nodding a ball down for Gabriel Jesus to touch onto ex-Red Philippe Coutinho to score.
Whether this limits his participation against Costa Rica remains to be seen.
Neymar could be a doubt for Brazil's Group E clash against Costa Rica on Friday after he limped out of a training session with what appeared to be a foot complaint on Tuesday. The introduction of Christian Bolanos gave them another rhythm after the break, with greater speed an attempt to provide some respite from the defensive onslaught. Their results coming into Russian Federation weren't great and they were lucky not to lose by more than a goal to nil against Serbia on Sunday. Costa Rica performed well but fell agonisingly short against one of the tournament favourites.
What a day for Brazil and the team's star Neymar.
7 - While it clearly was the late show from Brazil, the statistics back it up, too.
In truth, Brazil were well worth their victory today, but while it was Philipe Coutinho that made the difference against Costa Rica, Neymar, accidentally or otherwise, has once again pulled the spotlight on to himself with his actions.