MEXICO CITY (AP) -- Dustin Johnson didn't flinch when a challenge arrived out of nowhere Sunday, playing mistake-free over the final five holes to win the Mexico Championship in his debut as the No. 1 player in the world.
Johnson's four-shot lead going to the back nine turned into a one-shot deficit when Spanish rookie Jon Rahm ran off an eagle and two birdies at Chapultepec Golf Club. Johnson caught him with a birdie on the par-5 15th, and closed with three solid pars for a 3-under 68.
Rahm had gone 59 holes without a three-putt until taking two in a row at the worst time to fall back.
Johnson was steady right to the end to capture his 14th career victory on the PGA Tour, and his second straight. He reached No. 1 in the world two weeks ago with a five-shot victory at Riviera.
Johnson's last test was from a fairway bunker on the 18th, and he blasted that out to the middle of the green for a two-putt par and a one-shot victory over Tommy Fleetwood of England.
Johnson became the fifth player to win in his first tournament as No. 1 in the world. His fourth World Golf Championship title is second on the career list behind Tiger Woods at 18.
It was quite the consolation prize for the 26-year-old Fleetwood.
His 40-foot birdie putt on the final hole for a 66 put him alone in second and secured a spot in the Masters for the first time. He moves to No. 35 in the world and is certain to stay in the top 50 over the next three weeks before the cutoff to get an invitation to Augusta National.
Rahm's two late bogeys gave him a 68 and a tie for third with Ross Fisher (65).
Justin Thomas, who had a one-shot lead going into the final round in pursuit of his fourth PGA Tour victory this season, fell back with a double bogey from the water on the par-3 seventh and a bogey on the next hole from a bunker. He closed with a 72 and tied for fifth.