Boca Juniors extended its winless streak to nine league matches with a contentious 0-0 draw against Belgrano on Sunday, while city rival River Plate defeated Godoy Cruz 2-1 to stay within touching distance of the leaders in Argentina's top division.
Newell's Old Boys hold first place in the first division on 22 points after their 3-1 victory over Velez on Friday but are only one point clear of Lanus, which plays Monday against Union.
River's victory means it improves to 17 points in third position, while Boca is languishing in 17th place in the 20-team league with just one win from 10 matches.
Boca's players celebrated what they thought was the opening goal in the 2nd minute when Nicolas Blandi poked the ball towards the goal after a free-kick by Juan Roman Riquelme had rebounded off the post.
The refereree did not award a goal as it was too difficult to tell if Blandi's effort had crossed the line, but slow motion TV replays appeared to show that it had.
Neither Boca nor Belgrano had many chances in a tight game inside the famous Bombonera stadium, but both pressed for the victory as the clock wound down.
Guillermo Fernandez went through on goal with six minutes left, but sliced a right-foot shot wide of the post.
In the last minute, Lisandro Magallan headed just wide of the post from a free-kick swung in from the left, while Cesar Carranza almost scored for Belgrano moments later, but his shot was saved by Boca `keeper Agustin Orion.
River secured the vital win in the 54th minute when Colombian Eder Alvarez tapped in after goalkeeper Nelson Ibanez failed to collect a free-kick swung in from the right.
Manuel Lanzini had opened the scoring for River from the penalty spot in the 32nd, before Facundo Castillon finished for Godoy Cruz three minutes later after a defense-splitting through-ball by David Ramirez.
For Newell's, Martin Tonso opened the scoring with a curled left-foot shot in the 20th, with Marcos Caceres and Maximiliano Urruti adding second half goals after Ezequiel Rescaldani had equalized for Velez in the 57th.
Just after the final whistle sounded, Velez player Ivan Bello appeared to collapse. The player later said he'd had an epileptic seizure and that he is set to return to the field as early as next weekend.
In Sunday's other matches, San Lorenzo defeated Arsenal 3-1 and Rafaela won 2-0 against Independiente.
On Saturday, Quilmes overcame Estudiantes 1-0 and All Boys drew 1-1 with San Martin. Racing and Colon battled to a 1-1 draw on Friday.