Argentina entered the tournament on a 36-match unbeaten run and billed as one of the favourites for the title – a status they looked set to live up to when Messi slotted home a penalty in the 10th minute, during a first half the Copa America champions dominated and which saw three goals disallowed for offside.
However, the second half inside the Lusail Stadium was a different story entirely as a Saudi side which pressed superbly pulled level shortly after the break through Saleh Al Shehri. Only five minutes later, Saudi Arabia were ahead as Salem Al Dawsari struck the second with a superb curling effort.
A combination of heroic defending and a generally blunt Argentina attack saw Saudi Arabia produce one of the greatest shocks in World Cup history.
Reeling from a loss that threatens to derail their bid for a first World Cup triumph since 1986, Messi has demanded Argentina stick together and respond against Mexico in their next Group C match.
"We are a united group and now is the time to be united," the Argentina captain said. "We have to show that we are strong. We haven't suffered a defeat in a long time and now we have to prepare what comes against Mexico."
Messi's teammate, Angel Di Maria, admitted Argentina had been shell-shocked by the result, particularly after their three disallowed goals in the first half.
"We could easily have won 5-0, but they came two or three times and put them in," Di Maria said. "The offsides were always very close. We work on it and sometimes wanting to work before causes you to be ahead."
The goalless draw between Mexico and Poland – during which Robert Lewandowski missed a penalty – means the group is very much open, and Di Maria echoed Messi's stance by urging Argentina to return to winning ways in their next game.
"At some point it could happen," Di Maria said in reference to the end of Argentina's unbeaten run. "Now Mexico is coming and hopefully we will get the three points."