Lionel Messi's bid to win the World Cup got off to a nightmare start as he missed a penalty and his Argentina side were surprisingly held by debutants Iceland.
Messi was looking to match the impact his old rival Cristiano Ronaldo has made on the tournament - a sensational hat-trick against Spain on Friday night.
But instead of being the hero, Messi was the villain as his second-half spot-kick was easily saved by Hannes Halldorsson with the score at 1-1.
The draw was no less than Iceland, who are the smallest nation ever to play in a World Cup finals, deserved.
Their disciplined, defensive tactics frustrated an Argentina side who enjoyed the majority of possession and they also carried a threat going forward from the start, with Birkir Bjarnason scuffing a glorious early chance.
After surviving that scare, the thousands of Argentina fans who packed out the Spartak Stadium were dancing in the Moscow sunshine when Sergio Aguero put them ahead with a superb strike after 19 minutes.
But Jorge Sampaoli's side have a soft centre and their lead lasted only four minutes and 15 seconds, as Alfred Finnbogason equalised with his country's first goal at a World Cup finals.
Finnbogason had made a hash of a header seconds earlier but Argentina failed to clear and Willy Caballero could only palm Hordur Magnusson's shot into the path of the striker, who gleefully turned it home.
Messi's missed penalty was Argentina's only clear-cut chance in the second half, although Cristian Pavon's bouncing cross drew a fine late save from Halldorsson.
Messi could not provide the inspiration Argentina craved
All eyes were on Messi from the moment he walked into the Spartak Stadium, with the Argentina fans roaring their approval whenever his image flashed up on the big screen before the game, but he could not provide the inspiration they craved.
It was not for a lack of effort, or a shortage of attempts on goal, but the image of the Barcelona star that summed up his day was one of dejection, when he was left staring at the ground at the final whistle, with his hands on his knees.
He had been left to wander by Iceland, who did not try to man-mark him, but there were only flitting moments when he threatened to work his magic, with his best moment a first-half shot palmed away by Halldorsson.
Messi did provide the floated cross that led to Argentina's penalty after Maximiliano Meza was fouled by Magnusson, but he will want to forget what happened next.
Good save or not by Halldorsson, you do not expect Messi to miss from the spot on this stage.
He cannot afford to be as wasteful again if Argentina are to go deep into this tournament, especially with their fragile-looking defence.
Messi turns 31 later this month and is at his fourth World Cup, possibly his last chance for global glory to go with his spectacular achievements at club level with Barcelona.
If he is to lift Argentina to similar heights, and win his first international title, he will need some help.
Against Iceland, he got it from Aguero - who put Argentina ahead with a masterful finish.
The Manchester City striker brilliantly controlled Marcos Rojo's driven cross, then span away from his marker before burying his shot high in the net - his side's only quality moment in front of goal.
Iceland used to punching above their weight
The minnows had already tamed - not to mention annoyed - Ronaldo when they held Portugal in the group stage at Euro 2016, and this performance against Messi and co. was a reminder of how they reached the quarter-finals in France.
Heimir Hallgrimsson's side are used to punching above their weight, as England famously found out two years ago, and Argentina are simply their latest victims.
They arguably could have had more than a single point if they had made the most of their first half chances, the best of which was Bjarnason's miss when the ball ran into his path at the far post, but he failed to find the target.
The second half was more of a test of their renowned defensive resilience but if Argentina look to the individual brilliance of Messi to provide a spark, Iceland's strength is definitely as a collective unit.
Their fans were confident before the game, about both facing Argentina and getting out of Group D, and this performance more than justified their optimism.
Through sheer weight of numbers, Argentina's supporters did succeed in drowning out the famous 'Thunderclap' whenever it was aired at the Spartak Stadium, but it was the Iceland fans who were celebrating at the end.
Man of the Match - Hannes Halldorsson (Iceland)
Argentina play Croatia in Nizhny Novgorod on 21 June, while Iceland take on Nigeria in Volgograd the following day.
Messi's penalty woes
-Argentina failed to win their opening match at a World Cup tournament for the first time since losing to Cameroon in 1990. They eventually reached the final of that edition, losing out to Germany.
-This was Iceland's first ever World Cup game - Senegal were the last side to win on their debut in the competition (vs France in 2002), while Croatia were the last European side to do so (vs Jamaica in 1998).
-Iceland failed to record a single shot in the second half, while Argentina had 16 attempts on Iceland's goal in that period.
-Argentina have missed their last two penalties taken at a World Cup finals (excluding shootouts), with Ariel Ortega failing to convert against Sweden in 2002 before Messi's effort today.
-Argentina forward Messi had 11 shots in total in this match, more than he's had in any of his previous appearances at the World Cup.
-Javier Mascherano made his 144th appearance for Argentina, overtaking Javier Zanetti as their most capped player.
-Argentina's Aguero scored his first ever World Cup goal, in what was his ninth appearance across three tournaments in the competition.
-Finnbogason's goal was Iceland's first ever at the World Cup, with what was also their first shot on target in the competition.
-The Iceland forward's goal after 23 minutes was the earliest scored for a nation playing in their first ever World Cup match since Rashidi Yekini netted after 21 minutes for Nigeria against Bulgaria on June 21st 1994.
-Messi has missed four of the last seven penalties he's taken for club and country (excluding shootouts).