The American carded four birdies in a flawless round to hold a narrow advantage over Jordan Spieth.
He went through the first 12 holes in level par before picking up shots at the 13th, 14th, 16th and 18th to move on to nine under par for the tournament
Spieth went round in a three-under 68, with four birdies and a bogey, with a third American, Derek Ernst, on seven under following a 69.
Sean O’Hair was three shots off the pace in fourth place after shooting 69 ahead of a group of three on five under, Patrick Reed, Matt Kuchar and Henrik Stenson.
Scotland’s Russell Knox was the highest-placed Briton, in a tie for 10th on three under after a 70.
England’s Ian Poulter crashed completely out of contention, carding a four-over 75 to slip back to level par.