River Plate's title hopes for this season were ended after they were thrashed 5-1 at Buenos Aires rivals Lanus, who remain in the championship running with the win.
Goals from Christian Chavez, Guido Pizarro, Victor Ayala and Ismael Blanco had Lanus 4-1 up with just half an hour gone at the Estadio Cuidad de Lanus.
River could only manage Leonel Vangioni's sixth-minute strike in reply, and Blanco's second five minutes after the break sealed the emphatic defeat for Los Millonarios which leaves them six points behind leaders Newell's Old boys with just one game left to play.
"We didn't want our challenge to finish like this," River coach Ramon Diaz said after the match. "I apologise to our supporters for the way we played today. Lanus were ruthless for the whole 90 minutes."
Lanus are now also six points behind Newell's, but have a unique opportunity to make up the deficit and force a play-off.
On Wednesday, they will play the second half of their match at Estudiantes, which was abandoned at half-time last Monday after a Lanus fan was killed during clashes outside the stadium, an incident which led to a ban on away fans in all matches across the country.
Lanus will resume the match in La Plata at 2-0 down. However, if they can secure a win there and another at San Lorenzo on the final day of the campaign, then Newell's losing to Argentinos Juniors in their last match would see the two contenders finish level on points and force a play-off for the title.
The winner of that play-off will be determined the champions of the Torneo Final, and they will play Torneo Inicial winners Velez Sarsfield for the right to be crowned overall champions.