The first round of the elections took place on December 13.
A victory for Pinera would mean a right-wing government for the first time in Chile in 20 years and four governments.
The last right-wing government was the dictatorship of Augusto Pinochet, under which there were thousands of political killings and widespread human rights abuses.
At the end of campaigning Frei, 67, invoked a slogan of the first left-wing government to take power following Pinochet's rule.
"I want to invite all Chileans to change Chile's destiny with a pencil and paper," he called.
But Frei and Pinera, 60, have campaigned on generally similar policies, in particular on the need for economic growth and job creation.
Pinera, a Harvard-trained economist, whose business empire includes a significant stake in the main airline, ownership of the country's most popular football team and a principal television channel, has promised to double the country's $12,000 annual income per capita.
Frei, who was president from 1994 to 2000, did not any leave any significant impression on the electorate during his term.
But Bachelet has intimated that Pinera's business interests cast doubt on his suitability for the presidency.
"We are seeing a very content Frei and an insecure Pinera, who for the first time is realising that he can lose the presidential vote runoff," Bernardo Navarrete, a political scientist at the University of Santiago, said.