|WGC-Mexico Championship final leaderboard|
|-18 P Reed (US); -17 B DeChambeau (US); -15 J Rahm (Spa), E van Rooyen (SA); -14 R McIlroy (NI)|
|Selected others: -13 T Hatton (Eng), J Thomas (US); -11 P Casey (Eng); -6 T Fleetwood (Eng); -5 L Westwood (Eng); -3 S Lowry (Ire); level D Johnson (US); +13 G McDowell (NI)|
American Patrick Reed carded a four-under 67 to win the WGC-Mexico Championship by one stroke.
The 29-year-old world number 14, joint second overnight, could afford to bogey the last as he finished 18 under, one ahead of compatriot Bryson DeChambeau.
World number four Justin Thomas, one ahead at the start of the final day, slipped to a 73 for a share of sixth.
Jon Rahm's 67 left him three adrift in third place, with world number one Rory McIlroy a shot further back after a 68.
After nine birdies and a hole in one in a course record 61 on Saturday, Rahm birdied four of the first five holes to take a share of the lead and then managed to salvage par when his second shot found the water at the sixth.
By contrast Thomas, seeking his 13th PGA title, saw his tee shot flick off a tree into the water at the par-three seventh and he dropped out of the lead when another shot went at the next after his drive finished 80 yards off line.
His ball then ended behind the base of a tree at the 10th forcing him to play left-handed, and though he found a way through the trees back to the fairway, a double bogey resulted.
With other leading players failing to seize the momentum, DeChambeau, playing in the penultimate group with Rahm and McIlroy, produced seven birdies in nine holes from the sixth to move two clear.
The illustrious Rahm and McIlroy could only play the back nine in level par but the ever-tenacious Reed made two birdies to reduce the gap to one and was tied for the lead when DeChambeau left himself 63 feet away at the 17th and three-putted.
Despite a low sun causing visibility problems at the Club de Golf Chapultepec, Reed crafted an approach to two feet to birdie the 16th and take the lead.
DeChambeau, who led after two rounds, missed a 19-foot birdie chance at the last to finish with a 65 for 17 under, with Reed rattling in a third successive birdie after a long putt at the 17th.
There was still time for drama when Reed, taking an iron off the tee at the final hole, pushed it right into the trees but he found the green in three and safely two-putted for a five to add a second WGC title to his 2018 Masters victory.
Englishman Tyrrell Hatton, in his first event since November after wrist surgery, recorded five birdies in a 68 to share sixth.