Dustin Johnson was fighting off a string of challengers as the WGC-Mexico Championship headed for an exciting climax.
World Number One Johnson, whose place at the top of the Official World Golf Ranking was in jeopardy if Rory McIlroy could claim a third WGC title, began the final round a shot behind fellow American Justin Thomas.
Thomas doubled his advantage with a birdie at the first before dropping a shot on the second.
Playing partner Johnson birdied the same hole to draw level and picked up another shot on the par-five sixth, before Thomas found water with a terrible tee shot on the seventh to run up a double bogey.
Johnson was attempting to become the fifth player to win their first event since becoming World Number One, following in the footsteps of Ian Woosnam, David Duval, Vijay Singh and Adam Scott.
And the 32 year old was looking in imperious form as he birdied the eighth and ninth to extend his lead to four shots with just nine holes to play.
McIlroy was now six shots off the pace and not helped by seeing his drive on the ninth come to rest in an old divot.
Spain's Jon Rahm, who took out affiliate membership of the European Tour this week to become eligible for the Ryder Cup, closed the gap to three shots with an eagle from 14 feet on the 11th.
And when Johnson surprisingly three-putted the 12th from 25 feet, his four-shot lead had been cut in half, with Thomas Pieters and England's Tommy Fleetwood only three off the lead on 11 under.