Velez Sarsfield closed to within a point of River Plate at the top of the Clausura with a 2-0 win over 10-man Newell's Old Boys.
Velez led through Santiago Silva's seventh-minute goal and, after Marcelo Estigarribia was sent off for the visitor, Augusto Fernandez added a second.
Boca Juniors' struggles continued as they conceded two late goals to go down 2-0 at Lanus.
Diego Valeri got the first in the 79th minute before Santiago Hoyos added a second four minutes from time.
Boca has 10 points from nine games, and is only one point above Gimnasia La Plata, who drew 1-1 at Argentinos Juniors.
Gimnasia led through Jose Vizcarra's 25th minute goal, but Ciro Rius levelled four minutes later.
Two points behind Boca is All Boys, who won 1-0 at Tigre through Hugo Barrientos' ninth-minute strike.