Arnaud Montebourg, a lesser-known Socialist politician, came third with a surprisingly high 17 per cent of the vote.
His campaign focused on protectionism and tougher controls on financial markets and his relatively high score means that both Hollande and Aubry could veer to the left before next Sunday’s final round, in a bid to pick up his voters.
He is expected to announce his support for one of the two remaining candidates on television tonight.
Hollande and Aubry have much in common, they are both from the same political generation, and she replaced him as party leader in 2008.
She was highly critical of his legacy, maintaining he left a divided and rudderless Socialist party, and during the campaign she and others frequently hinted that he was lazy.
Aubry is the daughter of former European Commission president Jacques Delors, and is currently Mayor of the northern French city of Lille, but she is best known as the creator of France’s famous 35-hour-week, which she introduced as Employment Minister.
François Hollande has never held high office, though he has been a prominent figure on the left for years.
He has undergone a complete makeover in his bid to become France’s next president.
Previously known for his relaxed friendly style and good humour, he has adopted a more serious persona and shed 10 kilos.
He was the longtime partner of rival contestant Ségolène Royal, and is the father of her four children.
She was in tears when the results were announced, after coming fourth with only 7 per cent of the vote.