Duval shot 68 on Day 3 to move to 13-under-par 200 for the tournament.
The American star and reigning British Open champion is three better than fellow countryman Jerry Kelly (67), while Japans Katsunori Kuwabara (67) and Taichi Teshima (69) sit in joint third at 9-under.
Duvals already-strong lead might have even been greater, had it not been for a mishap at the par-4 14th, when a poor tee shot led him to attempt to hit a 7-iron approach from a lie encompassed by tree roots.
The result was a broken 7-iron, a double-bogey and eventually a 68 for a three-shot margin.
Said Duval after the round: I had a root in front of the ball, behind the ball, to the left of the ball and the right of the ball. I was basically in a box.'
News and Notes from the Dunlop Phoenix Open