Sao Paulo, Brazilian champions for the last three years in a row, made a losing start to the defence of their crown when they lost 1-0 at Fluminense on Sunday.
Midfielder Mauricio scored the winner from outside the box in the second minute, while former Olympique Lyon striker Fred hit the crossbar before halftime in front of a disappointing crowd of less than 15,000 at the huge Maracana stadium.
"There's nothing better than starting the Brazilian championship by beating the three-times champions," said Fluminense coach Carlos Alberto Parreira, who led Brazil at the 1994 and 2006 World Cups and was coach of South Africa until last year.
Striker Ronaldo sat out Corinthians' game as they lost 1-0 at home to Internacional on their return to the top flight after one season in the second division.
Internacional's winner was a memorable effort from striker Nilmar, who beat five opponents after latching onto Andres D'Alessandro's pass.
Ronaldo, who has scored eight goals since joining Corinthians after recovering from last year's knee injury, was rested for a cup game midweek.
Flamengo's new signing Adriano also missed his team's first game of the championship.
The former Inter Milan player, who signed last Wednesday, will start training next week.
Without him, Flamengo missed a penalty on their way losing 2-0 at Cruzeiro who played nearly the entire match with 10 men.
In the 14th minute, Cruzeiro defender Jancarlos saved a goal-bound header on the line and was sent off but Juan missed the resulting penalty for Flamengo.
Cruzeiro piled on the embarrassment for Brazil's most popular club when Kleber put them ahead from a penalty on the half hour and Ramires added another with one minute left.
Vanderlei Luxemburgo's Palmeiras opened their campaign on Saturday with a 2-1 win over Coritiba, Kerrison scoring the winner.