The National Civil Registry said 89.39 percent of the votes have been counted.
Local daily El Espectador on its website said that so far Zuluaga has 29.25 percent of the votes while Santos has 25.58 percent. Both failed to get an outright majority needed to avoid a second round.
Under the Colombian constitution, if no candidate wins more than 50 percent, the two leading contenders will go to a runoff in which the candidate who gets a simple majority of the votes wins.
Presidential elections were held Sunday in Colombia in a calm and peaceful environment in the 32 departments of the South American country.
A total of 32 million 975,158 citizens were registered to vote in the elections.
Also 559,952 people were registered to vote abroad in 217 polls located in 64 countries.