Brazilian President Dilma Rousseff has opened a lead over her top rival in their election race.

A survey by the Datafolha polling group released Tuesday night says 49 percent of voters would cast ballots for Rousseff in a second-round runoff ballot with rival Marina Silva. Silva is backed by 41 percent. A month ago Silva led Rousseff 50-to-40.

The poll says Silva's support in this Sunday's first-round election has slipped to 25 percent. That's five percentage points ahead of third-place Aecio Neves, but the gap between the two is narrowing.

A runoff vote on Oct. 26 is required if no candidate wins an outright majority this Sunday.

Datafolha interviewed 7,520 voters Monday and Tuesday across Brazil. The poll has a margin of error of two percentage points.