Nelson Vivas returned to La Plata only months after leaving his post as Estudiantes boss and while a huge slice of good fortune was needed his Defensa y Justicia side left the Estadio Único with three points.
Both sides had created chances but it was Estudiantes who had looked the more likely winners during the second half until the deadlock was finally broken in the 82nd minute. Fernando Márquez broke forward and shot from the edge of the penalty area but an enormous deflection diverted the ball past Mariano Andújar and into the opposite corner.
That proved enough to hand Defensa y Justicia an outstanding three points on the road.
In Córdoba, Belgrano earned a first win of the season after riding their luck against San Martín. The visitors had gone close in the first half with Nicolás Maná clipping the bar with a lob from distance but the game was ultimately decided in the 66th minute when Epifanio García scored from close range.
Following a dreadful last season and defeat on the opening weekend, victory comes as a welcome boost to Belgrano.