For the first time last week, polls put the environmentalist and Socialist Party candidate behind incumbent Dilma Rousseff in the event of a run-off, and put her neck-and-neck with Social Democrat Aecio Neves, who for weeks had been running a distant third.
But the latest poll by the Datafolha firm showed Silva was continuing to slip back, just when it matters most.
Silva only became the Socialist Party candidate after her running-mate Eduardo Campos died in an air crash. She got off to a strong start and a surge in support put her ahead in the polls, but she has since been losing momentum badly.
Brazil's first round of presidential voting is on Sunday, after which a run-off is likely.
Datafolha put Rousseff in front on 40 percent of first-round voter intentions, and Silva down a point on just 24, compared to 21 for Neves, who was up one.
That left the pair within the two-percent margin of error and Silva down 10 percent from a month ago.
Another poll Thursday by Ibope had Rousseff once again ahead in the first round with 40 percent, but saw Silva as having a five-point advantage over Neves.
Datafolha interviewed 12,022 people in 433 cities on Wednesday and Thursday. Ibope asked 3,010 voters in 205 municipalities Monday through Wednesday.