Newell's Old Boys caught up with Lanus on Friday when they won 3-1 at Velez Sarsfield, winners of the 'Inicial' championship in the first half of the season.
Lanus and Newell's have 22 points each from 10 matches with River Plate in third place on 20 after Sunday's 2-1 win away to Godoy Cruz in Mendoza.
River's centre back Eder Alvarez Balanta chested home their winner from a free kick in only his third start for the team, earning plaudits from coach Ramon Diaz.
Diaz compared Balanta with former River and Argentina captain Daniel Passarella, now the club's president, for his confident forays into attack.
"He has things of Passarella. He's strong, left-footed and he wants to stand out. He's helping us a lot," said Diaz, whose team are title challengers in their first season back from their humiliating first relegation in 2011.
Midfielder Manuel Lanzini put River ahead with a penalty and forward Facundo Castillon equalised in the 34th.
River had striker Carlos Luna sent off for a dangerous tackle in the final minute.
Union are 13th with 11 points but bottom of the relegation standings, a separate table based on teams' average points over three seasons with three teams going down in June.
The other teams facing a tough fight against relegation in the last nine matches of the 'Final' are San Martin and Independiente, record seven times South American champions who have never been relegated.
A change of coach in midweek with Miguel Angel Brindisi coming in for Americo Gallego did not have the desired effect as they lost 2-0 with 10 men at Atletico Rafaela, coached by one of their most noted former players, Jorge Burruchaga.
"We are our only rivals," said midfielder Rolfi Montenegro as the Red Devils continued to squander good scoring chances.
Another big club struggling for form are Boca Juniors whose 0-0 draw at home to Belgrano did little to help them get over a 6-1 defeat at San Martin the previous weekend.
To make matters worse, playmaker Juan Roman Riquelme came off after half an hour with a pulled muscle and looks set to miss the first leg of the Libertadores Cup last-16 tie at home to holders Corinthians on May 1, a repeat of last year's final.