Sebastian Pinera, a conservative billionaire former president who ruled from 2010-2014, is trying for a comeback to succeed center-left incumbent leader Michelle Bachelet, who is constitutionally excluded from standing again.
But his plan could be upset by Alejandro Guillier, a senator and veteran TV presenter who is Bachelet's candidate.
In the first round, on November 19, Pinera went in as the runaway favorite -- but then garnered a lower-than-expected 37 percent of the vote to Guillier's 22 percent.
Most problematically for Pinera, a surprising 20 percent of ballots went to a third-placed radical-left candidate, Beatriz Sanchez, and many could now go Guillier's way.
"The election will probably come down to a difference of less than 20,000 votes," said political scientist Marcello Mella at the University of Santiago.
Voting will begin at 8:00 am (1100 GMT) and end at 6:00 pm (2100 GMT), with official results expected within the following two hours.
With a possibly tight race before them, both candidates fiercely wooed the 13.4 million voters.
A high turnout would benefit Guillier, analysts said.
Pinera, who is worth $2.7 billion according to Forbes magazine, has painted himself as the most experienced steward of the economy.
"I'm not promising heaven and earth, but I promise that Chile will grow robustly," he said in a debate this week.
Though copper exports, which contribute greatly to Chile's wealth, are increasing thanks to demand from China and from the burgeoning manufacture of electric cars, the country is struggling relative to previous years.
Its GDP is forecast to expand a modest 1.4 percent this year, the slowest pace in eight years.
Pinera, 68, and Guillier, 64, are also promising to expand free university tuition brought in under Bachelet -- a measure with historical resonance in Chile because paid tuition was introduced under the 1973-1990 military dictatorship of Augusto Pinochet.
For Pinera, the vow was a U-turn, contradicting an earlier statement he made that "free things mean less commitment."
Bachelet's exit will be her second as president. She became the country's first female head of state in 2006. Pinera took over in 2010. Then Bachelet returned in 2014.
Whoever wins Sunday's election will take office in March 2018.
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