Fluminense, looking to lift the title for the second time in three years, are nine points ahead of Ronaldinho's second-placed Atletico with seven matches to go.
"It's an extremely important match, I think that's where the championship will be decided," said the 32-year-old Ronaldinho after Atletico's 2-2 draw away to Santos on Wednesday.
"If Fluminense win [against Atletico], there won't be anything to go for and if we win, we'll still be in it," the former world player of the year told reporters.
With Atletico, from Belo Horizonte, Ronaldinho looked on his way to winning the Brazilian title for the first time in his career but the team lost first place which they had held for more than half the championship while "Flu" have opened up a big lead in the final straight.
Ronaldinho, FIFA's world player of the year in 2004 and 2005, left Gremio at the age of 21 for Paris Saint-Germain in 2001.
However, even when the team were top, there was regular speculation about Ronaldinho's busy night life, his undoing at Flamengo, especially when the team's performances dipped in recent weeks.
