Tigre extended Newell's Old Boys' winless streak to five games with a 2-1 victory on Monday, which allowed San Lorenzo to stay top of the table following their 4-2 victory over Belgrano.
Juan Antonio Pizzi's San Lorenzo team moved to 30 points from 16 rounds, while Newell's remain on 29 after three draws and two defeats in their past five matches.
At San Lorenzo's Estadio Pedro Bidegain, the home side took the lead in just the third minute thanks to Emanuel Mas' thunderous strike into the roof of the net.
Belgrano drew level in the 60th minute when San Lorenzo's Pablo Alvarado put the ball into his own net but three minutes later the hosts were ahead again when Piatti won and converted a penalty.
San Lorenzo wrapped up their win on Saturday, and took a step towards the Torneo Inicial title, when Mauro Cetto volleyed home Leandro Romagnoli's 72nd-minute corner.
Tigre stunned Newell's on Monday after initially trailing the reigning Torneo Final champions.
Fabian Munoz put Newell's in front in the 20th minute but Tigre were level at half-time following Sebastian Rusculleda's strike 10 minutes before the break.
