A first-half goal from Gaston Fernandez gave Estudiantes a 1-0 win at San Lorenzo on Sunday to go five points clear at the top of the standings in the Argentine Apertura championship.
Meanwhile, a stoppage-time penalty from Agustin Pelletieri moved Lanus into the top half of the table with a 2-1 home win over Boca Juniors, whose inconsistent season continues.
Later Sunday, a late Adalberto Roman goal gave River Plate a 2-2 draw at Banfield as both teams lost ground with the leading pack.
Fernandez scored from an Enzo Perez pass in the 26th minute against San Lorenzo to move Estudiantes to 22 points from nine games, ahead of Velez Sarsfield and Arsenal.
''(The lead) is important, but there's still a long way to go and we can't ease off,'' Fernandez said. ''We know we've still got to improve.''
The defeat dented San Lorenzo's title hopes, with the club left on 15 points.
Earlier Sunday, Colon won 3-2 at Olimpo in a match between two teams in the bottom half of the standings.
Fernandez' strike against San Lorenzo was a worthy winner after Perez floated a ball into the box for the striker to control in a tight space. Fernandez dragged the ball back with the sole of his foot to send the goalkeeper the wrong way and slotted into the empty net.
San Lorenzo pushed forward more in the second half, but rarely troubled the Estudiantes defense and slumped to a second straight defeat after starting the season unbeaten.
Estudiantes won without injured captain Juan Sebastian Veron and will go into the second half of the Apertura tournament as the championship favorite.
At Boca's Bombonera stadium, Sebastian Blanco gave Lanus the lead after taking a short corner and advancing unopposed to the edge of the box before striking past Cristian Lucchetti in the eighth minute.
Juan Insaurralde equalized with a close-range header following a corner in the 34th, but Clemente Rodriguez fouled Eduardo Ledesma in the box to earn himself a red card and gift a penalty to Lanus.
Pelletieri smashed the spot kick down the center of the goal to win the match for Lanus with almost the final action of the game.
The defeat meant Boca has lost five games out of nine matches this season, with the club stuck in 14th position. The three points moved Lanus into the top half.
Elsewhere, Ruben Ramirez scored with a header to give Lanus a fourth-minute lead against River, before Ariel Ortega's penalty leveled the scores in the 63rd following Victor Lopez' shove on Mariano Pavone at a corner.
Lopez redeemed himself with a headed goal from a corner five minutes later, but Roman launched himself at a right wing cross to give River a share of the points.
After a result that helped neither side's title prospects, River has 15 points and Banfield has 14.
Earlier Sunday, Federico Higuain scored two first-half goals either side of an Ivan Moreno strike to put Colon three goals in front at halftime at Olimpo.
Olimpo came back in the second half with a Juan Tejera goal in the 64th, but Colon held on despite Diego Galvan pulling the score back to 3-2 just before the final whistle.
In results from Friday and Saturday, it was: Quilmes 0, Arsenal 1; Newell's Old Boys 2, Velez Sarsfield 0; All Boys 3, Tigre 3; Godoy Cruz 4, Independiente 1; Gimnasia 2, Argentinos Juniors 4; Racing Club 3, Huracan 0.
