In-form attacker Lucas Pratto was on target again as Velez made it four wins from four, edging Lanus 1-0 at the Estadio Jose Amalfitani on Thursday.
Pratto, who netted a brace against Independiente on Saturday, scored his fourth goal in three games with a strike seven minutes from time in Buenos Aires.
River Plate have dropped points previously but remain unbeaten following Wednesday's comfortable 3-0 victory over Defensa y Justicia.
Marcelo Gallardo's River drew their opening game but won back-to-back games prior to Defensa's arrival at the Estadio Monumental Antonio Vespucio Liberti.
And the Argentine powerhouses extended their winning streak courtesy of Carlos Sanchez's first-half double.
Colombian forward Teofilo Gutierrez sealed the points with 13 minutes remaining, directing a header into the top corner after Defensa's defence failed to cope with a ball across the six-yard box.
Racing Club, Newell's Old Boys, Godoy Cruz and Estudiantes also returned to winning ways.
Newell's Old Boys - fourth in the standings - almost squandered a three-goal lead but survived to win 3-2 at Atletico Rafaela on Wednesday.
Argentina international Maximiliano Rodriguez, Leandro Fernandez and Adrian Bastia's own goal helped Newell's to a comfortable first-half lead.
Fifth-placed Godoy Cruz got the better of rivals Rosario Central on Wednesday thanks to Ruben Ramirez's 69th-minute goal.
Estudiantes eased past struggling Boca, who are fourth from bottom, courtesy of Diego Vera's first-half brace and Joaquin Correa's opener.
First-half goals from Hector Villalba and Gonzalo Veron, and Nestor Ortigoza's stoppage-time penalty saw San Lorenzo to victory in Buenos Aires.
