The 25-year-old finished sixth in the javelin event Thursday evening and remains in first place heading into the closing 1,500-metre race, which takes place at 8:50 p.m ET.
Warner earned 761 points for his sixth-place finish in the javelin to give him a total of 7,879. He has a 286-point lead on Felipe Dos Santos of Brazil, meaning it will require a disaster in the final leg for Warner not to win gold.
Brazil's Alberto Luiz De Araujo is in the bronze-medal position while fellow Canadian Patrick Arbour is in sixth.
Warner, a London, Ont. native, had a throw of 61.53m on his second-last javelin attempt, which was his best in the event.
Grenada's Lindon Victor finished first in the event with a top throw of 66.97m, earning him 843 points.
Warner, who won a gold medal in the decathlon at the 2014 Commonwealth Games, finished fifth in the event at the 2012 London Olympics.