With 91 percent of the vote counted, Rousseff, the Workers' Party candidate and former Marxist guerrilla who enjoys the support of outgoing President Luiz Inacio Lula da Silva, received 46.7 percent while Serra, a former government minister who is the Brazilian Social Democracy party's candidate, had 32 percent, The Miami Herald reported. Marina Silva of the Green Party was third with nearly 20 percent.
Voters will go to the polls Oct. 31 to pick either Rousseff or Serra as their next president.
Arthur Ituassu, a professor of social communication at the Pontifícia Universade Catolica in Rio de Janeiro, viewed it as a defeat for Rousseff not to have won outright Sunday.
"But she is in a very strong position," Ituassu said. "... The chances are that will win in the second round."
However, Alexandre Barros of the Brazilian political consulting firm Early Warning Consulting says the runoff campaign means "we're going to have a completely new ball game."