Defender Ederson struck in the eighth minute before forward Luan doubled the visitors' lead 10 minutes from time on Thursday, reports Xinhua news agency.
The result at the Maracana Stadium lifted Gremio to 18 points from seven matches while Fluminense slipped to 10th with 10 points.
In Thursday's only other fixture, Bahia held Coritiba to a 0-0 draw in Curitiba after both teams were reduced to 10 men.
Coritiba's Kleber Gladiador and Bahia's Edson were sent off for an altercation in the 70th minute.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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