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.
Fluminense, led by Brazil striker Fred and former Portugal midfielder Deco, ended a 26-year wait by clinching the championship in 2010.
This season, with a 74 per cent success rate, they are clear favourites to regain the crown, especially if they do not lose to Atletico.
Ronaldinho has had his ups and downs since returning home to Brazil at the beginning of last year, a string of good performances for Flamengo even earning him a Brazil recall towards the end of 2011.
However, he failed to maintain his form and left Flamengo by the back door this year after winning only the Carioca (Rio de Janeiro) state championship.
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.
A World Cup winner with Brazil in 2002, he was at his brilliant best for Barcelona between 2003 and 2008, then had a spell with AC Milan before returning home.
At Atletico, he has built a good partnership with midfielder Bernard, a candidate for young player of the year, and former Everton and Manchester City striker Jo.
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.
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