A lone strike from substitute Cicero with just seven minutes remaining has given Gremio a slim advantage after the opening leg of the Copa Libertadores final on Wednesday.
The match in Porto Alegre seemed set to end in stalemate before Cicero, who had only been on the pitch for 11 minutes, pushed home the winner thanks to Jael's assist.
Argentina's Lanus, featuring in the Libertadores final for the first time, will host the return leg on Wednesday 29 November.
The winner will head to the United Arab Emirates early next month to represent South America in the 2017 FIFA Club World Cup.