Lanus was a 2-1 winner over San Martin San Juan to move level on points with Old Boys, snatching second on goal difference.
Diego Valeri put Lanus ahead after 33 minutes with Silvio Romero doubling the advantage from the penalty spot four minutes before the break.
Francisco Alarcon Cruz got one back for the visitor four minutes from time but Lanus hung on.
Lanus and Old Boys sit three points behind leader Velez Sarsfield.
Racing Club is three points further back after a 49th-minute strike from Luciano Vietto was enough to see off Godoy Cruz 1-0 away from home, the host's chances of a comeback dented considerably by the 66th-minute dismissal of goalkeeper Nelson Ibanez.
Arsenal Sarandi added to the woes of struggling Tigre with a 1-0 win courtesy of a Lisandro Lopez strike seven minutes before half-time.