Speaking to La Nacion newspaper, Elisa Carrio, Fernandez's closest rival, said: "I think there is a certain possibility of a second round. It depends on the undecided voters, on people voting for other candidates voting for us."
Roberto Lavagna, another candidate and a former economy minister who is credited with steering Argentina's economy back from its 2001 collapse, told the newspaper: "I don't believe the polls, I think many were paid for by the government and that Cristina de Kirchner is in freefall, especially in the cities."
Fernandez herself refused to make any last-minute comments to the media.
Throughout her campaign she gave hardly any interviews and she has remained vague on her policies.
She spent Saturday with her husband, Nestor Kirchner, who is stepping aside after one four-year term as president, in their home region of Patagonia in southern Argentina.
In her final speech on Thursday, she constantly referred to her husband's term and vowed to pursue his left-leaning policies.
She said: "Now we need to work on the remaining dreams."
Fernandez has brushed aside figures suggesting Argentina's economy is overheating with an estimated 20 per cent inflation, slow growth and low foreign investment.
Instead she has pledged to maintain hefty public spending and price controls, in the hope that the high worldwide demand for commodities that has benefited Argentina will continue.
The first lady has rebuffed critics who have accused her of being excessive in her designer clothes and use of heavy makeup, which have led some to compare her to Argentina's iconic Eva Peron.
"Should I disguise myself as a poor person to be a good political leader?" she asked in a rare exchanges with the media.
Fernandez's powerbase is among Argentina's poor and she has generated little enthusiasm among her country's business and monied classes.
Andres Saravia, a 32-year-old salesman working in a shopping centre in Buenos Aires, said: "The poor are the ones who will vote for her. Here in Argentina there is very little middle class [left after the economic crash]. They want things to stay the same."
Meanwhile, five of Argentina's leading opposition parties have joined together in an unprecedented move to monitor the presential election to ensure that all votes are fairly counted.
Omar Khalifa, Al Jazeera's correspondent in Buenos Aires, said the move came after Nestor Kirchner had hired a polling firm to conduct exit polls on Sunday.
Exit polls would begin to build a picture of the outcome of the election, before voting ends, potentially changing the result.
The union of parties plan to send representatives to about 3,000 polling booths around the country to prevent electoral fraud from affecting the election's outcome.
Earlier this week, La Nacion reported that officials from Kirchner's administration had said they might release results of the surveys while the polls are still open.
Argentine election laws forbid such a move.
Anibal Fernandez, the interior minister, said that it was a "healthy" idea for the opposition parties to monitor the election.
"I think it's healthy for them to do this. Each of the parties has the right to supervise the election and to send representatives to the voting booths," he said.
Khalifa said hundreds of people have spent hours queueing at governmental offices to get their identification papers in order to register their vote on Sunday.