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'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.
"I didn't feel like I putted my best, but I really hit the ball well," Johnson said. "I played just well enough, because I won by one."
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, who won 18 times since the series began in 1999.
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, who closed with three straight birdies for a 65. That assures Fisher a place in the next WGC event in three week at the Dell Match Play.
The great theater among the stars in Mexico City never really materialized.
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 with Thomas Pieters.
Rory McIlroy and Phil Mickelson, who started two shots behind, never got anything going.
Mickelson was wild with his opening tee shot and took bogey on the reachable par-4 first, and a double bogey on No. 8 ended his chances. McIlroy stayed in the mix until Johnson pulled away from him late on the front nine, and McIlroy made only one birdie on the back nine for the second straight day. They both shot 71.
Johnson looked as though he had this wrapped up when he made the turn.
He went long on the par-5 sixth, over the pin and up on a hill into the gallery, leaving what appeared to be a tough flop shot. The gallery ropes were down, and Johnson asked that the rope be removed from the ground so he could play a chip down the slope and onto the green. He pulled it off beautifully and made birdie, then rolled in a sliding 30-foot birdie putt on No. 8 and a 10-foot birdie on No. 9.
And then just like that, his lead was gone.
Rahm made an eagle putt from 15 feet on No. 11, and Johnson three-putted the 12th and made another bogey on the 13th from a bunker. They were tied at that point, and Rahm briefly went ahead with his birdie on the 15th.
That was as close as he got.
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. And except for a few nervous moments on the back nine, he looked just as dominant.
The greens of Chapultepec gave him fits all week. Johnson converted only 9-of-17 putts from inside 5 feet. In the opening round, he missed six putts from the 6-foot range or closer. His power and his clean striking allowed him to overcome that.
The most recent player to win in his debut at No. 1 was Adam Scott at the Colonial in 2014.
"It's a tough spot to be in," Johnson said. "There's a lot of pressure on you. I came out and played really well."
This article was written by Doug Ferguson from The Associated Press and was legally licensed through the NewsCred publisher network. Please direct all licensing questions to firstname.lastname@example.org.