If the unabashed economic reformer is anointed his party's candidate for the 2017 election, far-right leader Marine Le Pen stands to benefit.
He faces a second-round vote against another former prime minister, Alain Juppe, who trailed him by 15 percentage points.
So what's the vote for?
Mr Fillon - an outsider a few weeks ago - enjoyed a recent boost in popularity thanks to his image of authority and seriousness compared with Mr Sarkozy's more brazen demeanour. Such trenchant proposals may strike a chord with those faithful to Les Republicains but risk alienating swathes of other voters.
The case is one of several investigations to dog Sarkozy since he left office after what was dubbed a "bling-bling" presidency because of his flashy lifestyle.
Turnout was on course to be higher than in the first round of a Socialist primary in 2012, in which 2.7 million voted.
One voter, Emeline, said: "I am not at all on the Right, I'm very much on the Left and I really want to block Sarkozy and I know that the Left won't get to the second round (of the presidential election) so I just want to stop Sarkozy and Marine Le Pen". That's what happened on Sunday night.
The former president has been defeated in the conservative primary ahead of next year's presidential elections by Margaret Thatcher fan Francois Fillon.
For months, pollsters have been predicting Mr Juppe would win the primaries and subsequently defeat Ms Le Pen. But they were wrong.
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In a speech from his campaign headquarters in Paris, Mr Sarkozy called on his supporters to vote for Mr Fillon in the second round.
Juppe and Fillon have broadly similar programmes, underpinned by pledges to reinforce domestic security in a country still under a state of emergency following jihadist attacks that killed more than 230 people. And the shock: Mr Sarkozy is out; his political career is now over.
Mr Fillon served as Mr Sarkozy's prime minister during the latter's presidency between 2007 and 2012.
He was PM through the global financial crisis.
A snap poll by Opinionway after Sunday's results showed Fillon winning the head-to-head contest against Juppe with 56 percent of support. Mr Fillon and Mr Juppe also agreed on giving managers more flexibility by loosening the 35-hour weekly limit on employees' working time.
If this weekend's results are replicated, Mr Fillon will win.
The ruling Socialists and their allies will hold their own primaries in January.
So it's likely that their candidate will not make it past the first round of the presidential campaign.
The victor is expected to face far-right, anti-European Union National Front leader Marine Le Pen, and some analysts said the defeat of the deeply unpopular Sarkozy had lessened the prospects of her winning, easing investors' worries about a break-up of the euro zone.