River Plate suffered defeat on their return to top-flight action after going down 2-1 at home to Belgrano Cordoba in their Apertura opener.
River, the most successful side in Argentinian history with 33 titles, suffered relegation for the first time in their illustrious history last year but bounced back immediately to win promotion as second division champions.
However, the Buenos Aires giants could not get their Apertura campaign off to a winning start as they slumped to defeat against Belgrano - the team that beat them in the promotion/relegation play-offs a year ago.
The visitors struck twice either side of half-time through Lucas Melano (44) and Cesar Carranza (47) to take a 2-0 lead, and although River pulled a goal back through Manuel Lanzini in the 76th minute and then saw Cordoba lose Juan Olave to a red card in the 89th minute, the hosts were left empty-handed.
San Lorenzo also picked up a 2-1 win with goals from Leandro Romagnoli (17) and Franco Jara (45) putting them in control against San Martin San Juan, who could only net a late consolation through Sebastian Penco in reply.
Elsewhere, Godoy Cruz took a 13th-minute lead against All Boys through David Ramirez's penalty but were pegged back three minutes later by an Ivan Borghello equaliser as the teams shared a 1-1 draw.
