Fluminense were within one victory of being crowned Brazilian champions for the second time after a 2-1 win over Luiz Felipe Scolari's Palmeiras on Sunday.
Fluminense maintained a one-point lead over second placed Corinthians, who beat Vasco da Gama 2-0, and with one match to go they can lift their second title since 1984 with a home win over relegated Guarani next weekend.
Coach Muricy Ramalho, asked about his team facing a relegated side in their last match, said: "They (Guarani) are professionals, and that's something we mustn't doubt... We don't believe that we've already won it."
Cruzeiro, two points behind Fluminense, also remained in contention for the title with a 2-1 win over outgoing champions Flamengo. In their final matches, Corinthians face relegated Goias and Cruzeiro meet Palmeiras.
Corinthians could have done with a hand from their arch-rivals Palmeiras. Instead, as a reflection of the deep seated enmity between the two sides from Sao Paulo, when Dinei put Palmeiras ahead after five minutes against Fluminense their own fans jeered the striker.
With both sets of fans behind them, Flu equalised in the 19th minute through fullback Carlinhos.
Substitute Tarta, who came on for injured midfielder Deco during the first half, scored the winner 13 minutes after halftime when he picked up the rebound after goalkeeper Deola had blocked a shot from Emerson.
For Fluminense, considered the most elitist of Rio's four big clubs, to be on the verge of the title stands in contrast to last season when they only avoided relegation on the final day.
The man at the heart of Flu's return to the top is diminutive Argentine playmaker Dario Conca, who has played in all their matches in the championship.
He has also stood in as captain during long injury absences for striker Fred and former Chelsea player Deco, Flu's main recruit this season.
Fluminense, Corinthians and Cruzeiro have already secured berths in the 2011 Libertadores Cup, South America's champions' league.
Gremio and Botafogo are set to fight for a fourth berth at the former's home of Porto Alegre in southern Brazil next weekend in a match between the fourth and fifth-placed teams.
Scolari's Palmeiras had already suffered a disappointment this week when they were upset 2-1 at home by Goias in the second leg of their all-Brazilian semi-final in the Copa Sudamericana to go out on the away goals rule after a 2-2 aggregate result.
Goias will play in the Libertadores Cup next year as a second tier team if they beat Argentina's Independiente in the final of the Copa Sudamericana, the region's equivalent of the Europa League.
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