The Datafolha survey showed Silva with 46 percent voter support compared with 44 percent for Rousseff in a simulation of the expected second-round runoff. The gap has narrowed to two percentage points from four in the previous survey and is a statistical tie because it is within the poll's margin of error.
Rousseff has increased her support in the first-round vote to 37 percent from 36 percent in the previous Datafolha poll, while support for Silva fell to 30 percent from 33 percent. Centrist candidate Aecio Neves, the market favourite, has risen to 17 percent from 15 percent, Datafolha said.
If no candidate wins a majority in the Oct. 5 election, the race will be decided in a runoff three weeks later between the two top vote-getters.
The new poll surveyed 5,362 voters on Sept 17-18 and has a margin of error of plus or minus 2 percentage points. The poll was commissioned by TV Globo and the Folha de S. Paulo newspaper, which posted the results on its website.