Banfield, who beat South American champions Estudiantes, are just ahead on goal difference with both sides on 26 points after 12 matches.
Newell's, under former Argentina defender Roberto Sensini, won away to bottom team Tigre in Victoria on the northern outskirts of Buenos Aires.
Estudiantes, for whom captain Juan Sebastian Veron scored a fine equaliser from outside the box off the underside of the bar, have 23 points alongside San Lorenzo.
Newell's Uruguayan striker Joaquin Boghossian made up for an own goal by heading the winner five minutes from time at Tigre. He headed a cross into his own net in the 68th, just two minutes after midfielder Mauro Formica had given Newell's the lead.
In the first half, the 1.94-metres tall Boghossian helped ground staff tie up one side of the Newell's net after keeper Sebastian Peratta had fallen back into it and broken a stanchion tipping a lob from Tigre winger Ramiro Leone over the bar.
In the southern Buenos Aires suburb of Banfield, the Apertura championship's top scorer, Uruguayan Santiago Silva, put the home side ahead with his 11th goal in the 11th minute.
Estudiantes were handicapped from the 13th minute by the dismissal of winger Enzo Perez for swearing at the referee. Perez claimed he swore at an opponent.
Veron equalised in the 37th minute but Walter Erviti struck the winner in the 68th with a curling left-footed shot into the top far corner.
