SAO PAULO (Reuters) - Opposition candidate Aecio Neves has a shrinking lead over President Dilma Rousseff in Brazil's presidential race, an outlying poll showed on Friday, at odds with major pollsters that report a growing lead for the incumbent.

The new poll conducted by Sensus showed Neves with 54.6 percent of valid voter support and 45.4 percent for Rousseff. Sensus had shown an advantage of nearly 13 percentage points for Neves last week and almost 18 percentage points the week before.

Two more closely watched polls on Thursday showed Rousseff with a lead of 6 to 8 percentage points.

A mild economic rebound and a bruising campaign have boosted the president's campaign in recent weeks, but a crucial debate on Friday night could still shape voters' opinions before the final vote on Sunday.

The credibility of opinion polling in Brazil has taken a beating in this year's election. A flood of smaller surveys forecast results as much as 20 percentage points apart in the same week, and even big-time pollsters failed to capture the extent of Neves' support in the first round of voting on Oct. 5.

Sensus polled 2,000 voters in 136 cities from Oct. 21 to Oct. 24 for the survey published on Friday, which had a margin of error of 2.2 percentage points up or down.

(Reporting by Brad Haynes; Editing by Chizu Nomiyama)