Dustin Johnson didn’t flinch when a challenge arrived from nowhere, playing mistake-free over the final five holes to win the WGC-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 No1 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 placed 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 receive 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.

Best placed of the South Africans was Charl Schwartzel, despite a final round 75 that saw him finish on 284, some 14 shots behind Johnson and in a tie for 38th.

Louis Oosthuizen closed with a 68 for a two-over 286 and a share of 48th, while Sunshine Tour Order of Merit winner Brandon Stone was tied 67th after a final round 70 for 294. Richard Sterne had a second consecutive 72 for a 72-hole total of 299 and tied 72nd.

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