Second-placed Corinthians missed the chance to reduce the gap when after they had to fight back from two goals down for a 2-2 draw at home against Ceara.
Cruzeiro, in third, also drew 0-0 at home to Atletico Paranaense as only South American Libertadores Cup winners Internacional among the top seven won, beating Guarani 3-0.
Flu have 52 points, Corinthians 49, Cruzeiro 48 and Inter 44.
Corinthians scored twice in the final 19 minutes to salvage a point, their late equaliser an attempted cross from Argentine substitute Matias Defederico that sailed into the net.
"We shouldn't draw with Ceara," Corinthians' former Brazil fullback Roberto Carlos told reporters. "They are a team who were doing well but who lost form badly.
"We're fighting to be champions. We can't drop points like this."
Fluminense took the lead against Gremio Prudente on a water-logged pitch in Barueri after heavy rain when Rodriguinho finished a fine move instigated by Argentine midfielder Dario Conca on the left in the 34th minute.
The home side equalised through Vanderlei in the 73rd.
FLAMENGO DRAW
Title holders Flamengo, in crisis off the pitch and out of form on it, drew 1-1 with Botafogo in a Rio derby and remain in relegation danger in 15th place.
Botafogo went ahead with a free-kick from captain Lucio Flavio in the 35th minute and rival skipper Leo Moura equalised in the 75th from the rebound after goalkeeper Jefferson had parried Serbian Dejan Petkovic's penalty.
Both sides had a player sent off in the final quarter of an hour, Botafogo's Alessandro for a foul on Ronaldo Angelim that resulted in the penalty and Flamengo's Renato for a second booking two minutes later.
Flamengo, Brazil's most popular team whose director of football and former player Zico resigned this week over internal squabbles, are under their third coach this year, Silas, who could soon follow the man who signed him out of the door.
"If those people can do what they did to Zico, the greatest idol in the history of the club, they can do anything with me," said Silas.
"I have a contract and want to stay but I don't know what's going to happen."
