Justin Thomas made bookend bogeys but piled up eight birdies in between en route to a one-shot lead over Patrick Reed and Erik van Rooyen at the WGC-Mexico Championship, with Rory McIlroy a further four shots back ahead of the final round.
On a day when overnight leader Bryson DeChambeau self-destructed, Thomas carded six-under-par 65 at Club de Golf Chapultepec in Mexico City.
Two years after losing a playoff to Phil Mickelson, Thomas has a chance to avenge that near-miss, going into Sunday at 15-under 198.
Fellow American Reed (67) hit an unusually poor bunker shot to bogey the final hole and slip back into a tie for second with South African van Rooyen (67).
American DeChambeau struggled to a 71 and fell back into a tie for fourth with Spaniard Jon Rahm (61) and McIlroy (68).
The latter had five birdies in his round, though he will be disappointed with bogeys at six and 14.
Shane Lowry is at one under after his 71.
Rahm's course record include a hole-in-one at the 17th.