Guido Pizarro opened the scoring in the first minute of the game and substitute Diego Gonzalez added a second in the last.
It was enough to lift them ahead of Velez Sarsfield, who lost to Boca Juniors this weekend, on goal difference.
Luis Farina scored twice as Racing Club hammered Quilmes 4-0 to go third, three points behind the leading pair.
Adrian Centurion put Racing ahead in the 33rd minute, before Farina made the points safe with goals either side of half-time.
Jorge Serrano made life more difficult for Quilmes by being sent off in the 81st minute, and substitute Martin Guedes duly rounded off the scoring three minutes later.
The day's other game, between River Plate and Argentinos Juniors, finished goalless.