The clock is ticking for the incumbent President Nicolas Sarkozy as the campaign is to officially end by midnight Friday and his Socialist challenger Francois Hollande remains on top of all polls for Sunday's runoff.

The two rivals finished their final rallies respectively in Toulouse and Toulon late on Thursday, while the latest LH2 poll showed Hollande would gain 52 percent of votes against Sarkozy's 48 percent.

Sarkozy could become the first president to fail in a re-election bid since 1981, as well as the 12th eurozone leader to be forced out of office since the outbreak of sovereign debt crisis in 2009.

Earlier on Thursday, Sarkozy admitted that the result could be "very close" and "uncertain," while Hollande expressed satisfaction with his performance in the debate.

It was the first time that French people "saw me in a debate against the outgoing president," Hollande said in a TV interview on Thursday. He also further elaborated on his political and economic proposals, especially his foreign policy positions.

Sarkozy had tried but failed to throw a knockout blow to his rival in Wednesday night's fierce debate watched by nearly 17.8 million French voters, which could have been his last chance to prevail over his rival before the decisive polling.

LH2 opinion poll showed that only 41 percent of the respondents found Sarkozy more convincing in the debate, compared to Hollande's 45 percent and that there was no notable changes in voters' intensions.

Another setback for the incumbent came on Thursday night when centrist party leader Francois Bayrou, who had come fifth in the first round with 9.1 percent of votes, announced he would vote for Hollande, although he suggested his supporters should make their own choices.

Bayrou even gave his personal judgment on the two candidates in a press statement, in which he admitted that he did not quite agree with Hollande's economic plan but Sarkozy's chasing after the far right went against his values.

The two candidates are much divided over solutions for current economic difficulties, with Sarkozy calling for harder work and spending-cuts and Hollande emphasizing pro-growth initiatives.

The incumbent has been blamed for the country's economic gloom and often disliked for his "bling-bling" personality. After the first round, he was also caught in a dilemma of risking centrists' votes while courting the far-right.

Earlier this week, the far-right leader Marine Le Pen, who had surprisingly won 18 percent of votes in the first round, said she would cast a blank vote while slamming Sarkozy's call for border controls as "pure theatrics."

With merely 25 hours left for Sarkozy's uphill battle, the chance of winning is undoubtedly slim if not zero, analysts say.