Voter surveys show about half of her supporters would back Sarkozy in a second round and perhaps one fifth would vote for Hollande, making her a potential kingmaker in the runoff.
Jean-Marie Le Pen's 16.9 percent score in the 2002 first round caused a political earthquake, knocking then Socialist Prime Minister Lionel Jospin out of the runoff and forcing left-wing voters to rally behind conservative Jacques Chirac.
Sarkozy, 57, has painted himself as the safest pair of hands to lead France and the euro zone in turbulent times, but Sunday's vote appeared to be a strong rejection of his flashy style as well as his economic record.
If Hollande wins on May 6, joining a small minority of left-wing governments in Europe, he has promised to lead a push for a bigger focus on growth in the euro zone, mainly by adding pro-growth clauses to a European budget discipline treaty.
The prospect of a renegotiation of the pact is causing some concern in financial markets, as is Hollande's focus on tax rises over austerity at a time when sluggish growth is threatening France's ability to meet deficit-cutting goals.
France's sickly growth, along with its stubbornly high unemployment, are major factors hampering Sarkozy's battle to win a second term, despite an energetic campaign against the blander but more popular Hollande.
Sarkozy would be the 11th euro zone leader to be swept out since the start of the bloc's debt crisis in late 2009 and the first French president to lose a re-election bid in more than 30 years. A deep dislike of a manner many see as arrogant and too informal has also driven many people to vote against him.
"France needs a radical change of direction, mainly on the economy," said Jean-Noel Harvet, a public sector worker voting earlier on Sunday in the northern town of Cambrai.
Hollande, 57, promises less drastic spending cuts than Sarkozy proposed and wants higher taxes on the wealthy to fund state-aided job creation, in particular a 75 percent upper tax rate on income above 1 million euros ($1.32 million).
He would be only France's second left-wing leader since the founding of the Fifth Republic in 1958, and its first since Francois Mitterrand, who beat incumbent Valery Giscard-d'Estaing in 1981 and ruled until 1995.
Hollande had called on his supporters to take nothing for granted, mindful of the fiasco for the left in 2002 when record abstention saw the Socialist Jospin pushed out in the first round by the elder Le Pen.
Turnout ended up at a healthy 70.6 percent three hours before polls closed, just below 73.9 percent recorded in the 2007, which was the highest in two decades.
Intellpuke: You can read this Reuters article in context here: worldnews.msnbc.msn.com/_news/2012/04/22/11331803-sarkozy-hollande-advance-in-french-vote-far-rights-le-pen-gets-20-percent?lite