The much-fancied Argentines, second favourites to lift the trophy going into their Group C opener in Qatar, were in total control at the break after Lionel Messi opened his account from the penalty spot.
But the Gulf state were transformed after the interval and flipped the game on its head with two goals in the space of five minutes, Saleh Al-Shehri levelling with a low effort and Salem Al Dawsari scoring a peach of a winner.
What: World Cup
When: Sunday 20th November – Sunday 18th December 2022
How to watch: BBC & ITV
There was absolutely no sense that we were on the cusp of watching one of the World Cup's greatest upsets when Lionel Messi put the Albiceleste in front from the penalty spot.
The 35-year-old, who had already drawn one save from the keeper before his opener, was at the heart of everything good about Argentina as they sought to put their 51st-ranked opponents to bed by half time.
They had another three efforts disallowed for offside as they repeatedly tried to break Saudi Arabia's high line, and those misjudgements would go on to prove costly.
Within 10 minutes of the restart, the Saudis, still gambling defensively but more proactive in attack, had beaten Emi Martinez twice leaving the shellshocked Argentines half an hour to avert a disaster.
And that's when their cool went, their class disappeared and they suddenly looked ordinary outfit against an opponent – 14/1 shots lest we forget – who were throwing bodies everywhere in an heroic effort.
Argentina doubtless never thought they'd rue those disallowed first-half goals.
On top of that, Messi was fit and ready to spark the world's second-ranked team into life, overpowering the Saudis ahead of supposedly sterner tests against Poland and Mexico.
What was astonishing, however, was that having cut the Saudis apart time and again in the first 45 minutes, they did it hardly at all in the second period as the inspirational well ran dry.
Parallels with 1990 are obvious. Back then Argentina went into their opener at Italia '90 as the defending champs and duly lost 1-0 to a Cameroon side, who kicked them into submission.
The Saudis weren't quite that ruthless but they didn't back off as a haul of six yellow cards testifies, before they were able to celebrate their greatest ever result which saw them cut to to win the tournament.
Argentina are now 8/1 ahead of a daren't-lose showdown with Mexico on Saturday night that has suddenly become critical. Lose that and their World Cup could be over less than a week after it had started.
Despite the defeat, Lionel Scaloni's South American champions were still to qualify from Group C and 13/8 to win it – (the Saudis, should you be tempted after their magnificent first effort, are 7/2 to win the section).
But many punters and fans were asking at the final whistle whether this might be one challenge too far for the 35-year-old – yes, 35 – who has carried the hopes of his nation on his shoulders so manfully for a decade and more.