Gonzalo Martinez's stoppage-time penalty saw River Plate past Gremio and into the Copa Libertadores final.
A second-half header from Michel was enough for Gremio to edge River Plate 1-0 in the Copa Libertadores.
River Plate and Gremio will meet in the final four after respective wins against Independiente and Atletico Tucuman.
Gremio travelled to Argentina and drew first blood in the opening leg of their quarter-final tie thanks to Alisson and Everton.
Copa Libertadores holders Gremio suffered a narrow 2-1 loss away to 10-man Estudiantes in the round-of-16 first leg on Tuesday.
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Gremio midfielder Arthur will move to Barcelona in summer, but he is sad to miss out on his dream of playing with Andres Iniesta.
The midfielder played 90 minutes of Gremio's Copa Libertadores match against Monagas and certainly showcased his talent, providing a great assist to Luan to score the Brazilian side's third goal of the night.
Boca Juniors opened their 2018 account with a 1-0 win at home to Junior in Buenos Aires on Wednesday, while Gremio and Penarol also won.
Renato Portaluppi, Arthur's former coach at Gremio, believes the midfielder has a big future ahead of him both at club and international level.