Brazilian President Dilma Rousseff continues to broaden her advantage ahead of Sunday's election, but it is not clear which of her rivals she will face in an expected second-round runoff, a new poll showed on Friday.

Environmentalist Marina Silva, who was favored to defeat Rousseff after a surge in the polls one month ago, has steadily lost support and is now running neck-and-neck with market favorite Aecio Neves in a battle for second place, a survey by pollster Sensus said.

The poll showed Rousseff gaining 2 percentage points to 37.3 percent from the previous Sensus survey a week ago, while Silva has fallen to 22.5 percent from 25 percent. Neves held his ground at 20.6 percent.

The gap between Silva and Neves is within the poll's margin of error of 2 percentage points up or down and statistically insignificant.

Sensus polls are not as closely watched by political analysts as the surveys by bigger research firms Datafolha and Ibope that are more frequent and use larger samples.

But the poll confirmed a Datafolha survey on Thursday that also showed Neves within striking distance of second place.

If no candidate secures a majority in the first round, the top two vote-getters will face off on Oct. 26.

In a turnaround to the hotly contested presidential race, Rousseff has emerged as the favorite to gain a second term, with most polls showing her winning in a runoff vote.

Rousseff's recovery has weighed down on markets where investors had hoped that Silva could oust the incumbent and bring in more business-friendly policies to restore confidence and revive Brazil's stagnant economy.

Citibank analysts in Brazil revised their forecast in a note to clients that said Silva was no longer the favorite. "We now see the incumbent president, Dilma Rousseff, being re-elected with 67 percent of chances," the note said.