Teofilo Gutierrez scored a 19th-minute penalty to sink visiting Olimpo and move Racing Club up to second in the table, a point behind the pacesetters.
Tigre's 3-1 win at struggling Estudiantes was based on an impressive first half performance.
The visitors raced into a three-goal lead inside 24 minutes with goals from Carlos Luna, Diego Morales (penalty) and Roman Martinez, with the home side having goalkeeper Damien Albil dismissed at 1-0.
Estudiantes, second from bottom and winless after six matches, needed a 61st-minute own goal from Carlos Casteglione to get off the mark - but they were not going to stop Tigre moving up to fourth.
Godoy Cruz are up to seventh after a come-from-behind 6-1 defeat of All Boys.
Eduardo Dominguez gave the visitors the lead midway through the first half but Ruben Ramirez and Emanuel Perea's own goal turned things around by half-time and Ramirez, Leandro Caruso and Diego Villar made the points safe.
Dominguez's evening took a turn for the worse on the hour when he was dismissed, with substitute Jorge Curbelo completing the scoring for the home side four minutes from time.