Rousseff, a leftist from the ruling Workers' Party, would receive the vote of 38 percent of those surveyed by pollster Ibope, compared with 40 percent during a previous poll in May.
Aecio Neves, the senator from the centrist PSDB party who is Rousseff's biggest rival, was backed by 22 percent of those polled, up from 20 percent previously.
Eduardo Campos, governor of the northeastern state of Pernambuco and the candidate for a rival leftist party, polled at 13 percent, also up two points from 11 percent in May.
Despite what remains a comfortable lead, Rousseff's approval ratings over the past year have suffered because of a stagnant economy, persistent inflation, and protests over public spending on the upcoming World Cup of soccer and a series of scandals at the state run energy company, Petroleo Brasileiro SA, or Petrobras.
The Ibope survey polled 2002 people between June 4 and June 7. The poll has a margin of error of plus or minus two percentage points.