Argentina came from behind to draw 1-1 with Venezuela at home on Tuesday, a result that leaves the 2014 losing finalists a long way from guaranteeing their place in Russia next year.
The hosts had won all six of their previous home qualifiers against Venezuela and were expected to do the same against the team at the bottom of the South American group and without a win in 10 games.
But Argentina's failure to take all three points left them fifth in the standings with two matches remaining - out of the top four spots which brings direct qualification for the finals.
Venezuela got the opening goal just four minutes into the second half.
Jhon Murillo ran onto an incisive through ball and expertly chipped the advancing goalkeeper to give the visitors a surprise lead.
The advantage, though, lasted just four minutes when a cross was knocked into his own goal by Rolf Feltscher.
Mauro Icardi had a good shout for a penalty waved away after an hour but Argentina, with Lionel Messi not at his commanding best, looked lacklustre throughout and were unable to stamp their authority on the match.
Neither they nor Venezuela created many clear-cut chances and the home side trooped off to the boos of their disgruntled fans.
Brazil top the 10-team group and are the only South American side so far to have qualified for Russia 2018.
They drew 1-1 with Colombia in a hard-fought qualifier after Radamel Falcao's second-half header cancelled out Willian's stunning first-half volley.
Colombia have still never beaten Brazil in a World Cup qualifying match but the draw marked the first time in 10 games that Brazilian coach Tite has come away from a qualifier with anything other than all three points.
Uruguay, who won 2-1 away at Paraguay, are in second-place with 27 points, followed by Colombia on 26.
Peru, who have not played in the World Cup Finals since 1982, beat Ecuador 2-1 earlier in the evening to move into fourth place with 24 points, ahead of Argentina on goal difference.
The fifth-placed side go into an intercontinental playoff against New Zealand.
