It confirmed the five-point lead Rousseff had in a June 23 survey by polling firm Ibope.

Brazil will hold elections on October 3 to choose a successor to President Luiz Inacio Lula da Silva.

The 62-year-old Rousseff, Lula's former chief of staff, has taken the lead bolstered by a booming Brazilian economy and the president's wide popularity.

Brazil's economic growth of 9 percent in the first quarter is the nation's fastest pace in 14 years.

A previous Vox Populi poll in May showed Rousseff with 38 percent of voter support against Serra's 35 percent.

Brazil's presidential race will not move into high gear until the soccer World Cup is over in mid-July.

Lula, one of Brazil's most popular presidents, cannot run again due to a two-term limit, and he is urging voters to make Rousseff the South American country's first female leader.

The Vox Populi poll has a 1.8 percent margin on error and was conducted between June 24 and 26 among 3,000 potential voters.

**Reporting by Maria Carolina Marcello; writing by Reese Ewing; editing by Anthony Boadle