Saudi Arabia pulled off one of the great upsets in World Cup history this morning, beating widely-fancied Argentina 2-1 to bring the tournament to life and leave Lionel Messi facing the prospect of a humiliating exit at his final World Cup.
The two-time winners led through Messi, who is aiming to be world champion at the fifth attempt, but now face a battle to reach the knockouts after a remarkable second half from the Saudis.
Messi stroked home a 10th-minute penalty after Leandro Paredes was bundled over but Saudi Arabia stunned Argentina with two quick-fire goals at the start of the second half - sparking a phenomenal atmosphere inside the 80,000-seat Lusail Stadium, which will host the Final.
On this evidence, Argentina are unlikely to be back on December 18, despite arriving in Qatar among the pre-tournament favourites and aiming here to match Italy's record of 37 international games unbeaten.
There was a sense that Lionel Scaloni's side still had to prove themselves against the best in Europe but no-one predicted they would struggle against the Group C underdogs.
Saudi Arabia took full advantage of Argentina frustration at having three first half-goals disallowed to put in a display for the ages -- their tireless running and last-ditch defending at 2-1 just as impressive as two superbly-taken goals.
Three minutes after the interval, Saleh Al-Shehri beat Tottenham's Cristian Romero to the ball and fired a brilliant, low finish across goalkeeper Emiliano Martínez to equalise and send the green wall into flag-waving delirium.
Argentine were rattled, the frustration at their earlier disallowed goals now turned to outright panic.
Sensing blood, Saudi stayed firmly on the front foot. Romero got his head in the way of a goal-bound effort but the ball was picked up by Salem Aldawsari, who stepped inside and curled a superb effort into the top corner.
It would be a contender for goal of the tournament, even without the remarkable context.
Scaloni had seen enough and ripped up his side with a triple change, Romero among the players withdrawn and Manchester pair Lisandro Martinez and Julian Alvarez entering the fray
Argentina recovered an element of poise and Messi was denied by a phenomenal last-ditch tackle before Mohammed Alowais saved point-blank from Nicolás Tagliafico.
It promised to be a long final half-hour for the Saudis but even as Hervé Renard's side tired, Argentina lacked finesse and conviction in the final third, underlined when Messi uncharacteristically sent a free-kick well over the bar before heading tamely at the goalkeeper.
The 35-year-old, who has said this will likely be his last World Cup, was clearly the main attraction and started the game brightly, and saw a trademark low shot well-saved by Alowais.
Argentina thought they had doubled their lead three times before the interval but Messi and Lautaro Martínez -- twice -- were ruled fractionally offside after racing clear of the Saudi defence.
Both Martinez's disallowed goals were special, the first a dink over the onrushing Alowais, who was then sold a brilliant dummy for the second after the Inter Milan striker had been sprung again from Messi's pass.
Argentina's early penalty came after the referee consulted the pitch-side monitor but will raise further questions about consistent application of the laws at the tournament, given it was a less compelling claim than Harry Maguire's appeal in England's 6-2 win over Iran yesterday.
Argentina were dysfunctional and pedestrian in Russia four years ago and here they were just too easily unsettled by a proactive, high-pressing and well-organised Saudi side. By half-time, they had wracked up more offsides, seven, than in the entire 2018 World Cup.
Saudi Arabia, who lost captain Salman Al-Faraj to injury in the first half, will now fancy their chances of qualifying from a group containing Mexico and Poland, while the Argentines must lick their wounds and recover from an historic failure.