Holders Fluminense were upset 2-0 at home by Santos while Corinthians beat Gremio 2-1 away thanks to a fine performance from striker Liedson.
Ronaldinho, who had disappointed Flamengo fans with his indifferent form since joining from AC Milan in January, scored a brilliant goal and laid on another for Diego Mauricio with a one-two inside the box.
Modest Avai fielded a second-string side ahead of their Copa Brasil semi-final on Wednesday.
"That's as good as it gets. To return scoring a goal, laying on another after a long time is marvellous," Ronaldinho told reporters.
"I'm going to try to go after the conquest that's missing from my curriculum," added Ronaldinho, who had been criticised and jeered by fans after Flamengo's Copa Brasil elimination earlier in May.
LUCAS SCORES
Brazil teenager Lucas and striker Dagoberto scored the goals for Sao Paulo, who won the Brazilian championship three years in a row between 2006 and 2008 but have not been in form this year and lost Brazil 2010 World Cup striker Luis Fabiano to injury.
Dagoberto opened the scoring in the 34th minute, finishing a good move involving Casemiro and Lucas, who increased Sao Paulo's lead four minutes after the break with a solo run and shot into the roof of the net.
"We want to get rid of that poor phase we've had in the first half of the year and from here on it's only victories. Sao Paulo are a big team, with tradition and only we players can charge our situation," Lucas told reporters.
Fluminense midfielders Deco and Argentine Dario Conca, voted best player in Brazil last season, had little impact on the game.
At the Olimpico in Porto Alegre, Gremio went ahead just before the hour with a Douglas penalty but within 15 minutes Liedson had turned the match round for Corinthians.
Captain Chicao converted a 66th-minute penalty after a foul on Liedson, who then volleyed home the winner from a throw-in.
Santos, resting several leading players for Wednesday's Libertadores Cup semi-final first leg against Paraguay's Cerro Porteno, drew 1-1 at home to Internacional, while Cruzeiro went down 1-0 away to promoted Figueirense.
