Messi's fans say Ronaldo, at 33, is slowing down.
Aguero, with his back to goal on the penalty spot, stopped a hopeful Marcos Rojo daisy-cutter with one touch and swivelled to smash a left-footed drive into the top corner on 19 minutes.
Iceland coach Heimer Hallgrimsson's tactical masterstroke was partly achieved by a surprise selection of goalscorer Alfred Finbogason, who quickly cancelled out a brilliant first half opener from Manchester City's Sergio Aguero. Argentina, for just about as long as can be remembered, has spent major tournaments toiling away against deep-lying opposition, waiting always for Lionel Messi to produce a moment of genius to unpick the defense. The ref pointed to the spot and naturally Messi was decided amongst his teammates he was the worthy choice to play "God".
Iceland goalkeeper Hannes Halldorsson made a save on a penalty shot by Argentina's Lionel Messi.
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But scoring goals was never going to be the main problem for this side, the question was whether they could avoid conceding them. Could this be yet another disappointing tourney for the world's greatest player or does Cristiano Ronaldo stake his claim?
Messi had two more chances as the match stretched into its final 10 minutes.
Nine minutes from the end, Messi curled a superb strike narrowly wide from the edge of the D, and Argentina threw Gonzalo Higuain into the fray as they pressed for a late victor. "It's a very even World Cup and you're seeing that, because all the games are very close", said Messi.
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"We have things to improve, we're going to beat Croatia".
That was until the 64 minute, when luck seemingly fell in Argentina's favour as they were awarded a penalty following a clumsy challenge. Seldom threatening to take the lead itself, Iceland also shut down Argentina's efforts to win the game.
Two years ago, Messi had missed a penalty that cost them the Copa America title and he had immediately announced his retirement from global football, it had hurt him that much. Like the FC Barcelona's superstar, who was not born yet the last time Argentina secured the World Cup championship title, the door may be also closing for other star players like Neymar, who will try to lead the Brazil team to its sixth World Cup title?
But with trademark dark humor, the part-time coach and full-time dentist, conceded: "We wanted to use spaces behind them - but it's hard when you don't have the ball".
Croatia and Nigeria won't be easily beaten, but neither opponent will have been encouraged by what they've seen from Iceland here, and they could make history again by progressing.
Eleven shots, no goals - Messi struggles after Ronaldo fireworks
Johann Beg Gudmundsson was forced off injured after becoming the first Claret to play at a World Cup Finals in 36 years. As time wore on, Messi wore the look of a beaten man as attack after attack was broken down by Iceland .