According to Reuters, Neves made a jump to second place, partial results have shown.
Three-quarters of the results have been counted, Rousseff led with 40.4% of the vote while Neves held 35.2%.
Rousseff was up against environmentalist Marina Silva, who was recently ahead in the poll. Silva dropped to third in the polls, taking in just 21% of the votes.
Reuters said results were not yet in from the poorer states of the nation where Rousseff was stronger.
With no candidate coming out on top, the first two finishers will face each other in a second round of voting on 26 October.