A Ramiro Funes Mori own goal saw Racing Club beat River Plate 1-0 at the Estadio Presidente Juan Domingo Peron.
It extended River's winless run to five matches in all competitions, as they gave up the Primera lead and slipped two points behind top spot.
With two matches remaining, Racing are well-placed to claim the title.
They needed a bit of luck for their goal in the 16th minute when a goalmouth scramble led to a deflection off Mori going past his own goalkeeper.
And Racing have proven they only need one goal, holding on for their fourth consecutive 1-0 victory.
Lanus are also in title contention, sitting two points off top spot after a 2-0 win at home to Gimnasia La Plata.
Silvio Romero struck with a close-range volley in the 20th minute and Victor Ayala sealed the win with a strike from distance in the second half.
Elsewhere, Banfield claimed a come-from-behind 3-1 win at home to Quilmes.
Lucas Albertengo struck a second-half brace for Atletico Rafaela, who were 2-0 winners over San Lorenzo.
Jaime Ayovi's double in the first half set up Godoy Cruz's 4-1 thrashing of Velez Sarsfield.
A pair of goals from Jonathan Calleri helped Boca Juniors past a 10-man Independiente 3-1.
Olimpo and Rosario Central drew 1-1, Tigre rallied late to overcome Belgrano 2-1 and Estudiantes drew 0-0 against Defensa y Justicia.
On Monday, Maxi Rodriguez struck a brace for Newell's Old Boys but they went down 4-2 at home to Arsenal.
