FedEx Cup holder Rory McIlroy might have hoped to deal Dustin Johnson another psychological blow, but he came up four shots short as the world No 1 consolidated his position at the top of game with a one-shot victory in the WGC-Mexico Championship in Mexico City.

Only Tiger Woods has won more World Golf Championships that the 32-year old American, whose final round challenge at Club de Golf Chapultepec did not come from McIlroy or overnight leader Justin Thomas but Spain's Jon Rahm and eventual runner-up Tommy Fleetwood.

"I needed to get off to a fast start and I didn’t," said McIlroy, who shot a level par 71 to finish tied for seventh, four shots behind Johnson in his first start after 44 days out with a rib injury.

"That was really it. I mean you look at what DJ did through sort of the first nine holes, I needed to get off to a start like that."

Rahm, the 22-year old Basque who played aggressively to claim a maiden PGA Tour win in January’s Farmers Insurance Open, moved into a one-stroke lead with three holes to play when he eagled the 11th and then birdied the 14th and 15th to sneak one ahead of Johnson on 14 under par.

US Open champion Johnson began the final day one shot behind Thomas, who eventually shot a one-over 72 to share fifth with Belgium’s Thomas Pieters on 11 under with an uninspired McIlroy (71) tied for seventh with Phil Mickelson (71) and Brandt Snedeker (65) on 10-under.

But having gone to the turn in four under 31 to lead by four shots from Pieters and by six from a sluggish McIlroy, Johnson bogeyed the 12th and 13th from nowhere to suddenly find himself trailing Rahm by one.

He quickly birdied the par-five 15th to draw level, then looked on as Rahm's 59-hole streak without a three-putt ended with back-to-back three-putt bogeys from long range at the 16th and 17th.

Rahm eventually signed for a 68 to finish tied for third with Ross Fisher, two shots behind Johnson on 12-under par as England’s Fleetwood shot 66 to set the target at 13-under and eventually claim second place and a cheque for $1,045,000.

Johnson needed a par four at the last to win his 14th PGA Tour title and $1.66m. But having driven into a fairway bunker, he hit what he reckoned was the best shot he hit all week to 18 feet and two-putted from 18 feet for victory.

"I played really well all week long," said Johnson, who won in his last start at the Genesis Open in Los Angeles. "I was really proud of how I hit the ball."

Johnson knew that if McIlroy triumphed, he could finish no worse than third to hold onto the world No 1 ranking for a third successive week and avoid losing out again to the Ulsterman, who won the Tour Championship last year to take the FedEx Cup from him at the death.

But Johnson did far better than that,  and with his win, he now has more World Golf Championship victories (four) than any other player bar 18-time WGC winner Woods.

“It’s a tough spot to be in,” Johnson said the responsibilities that come with the number one ranking. "There are a lot of pressures on you and I came out and played really well.

"I made the turn with a nice lead and with those bogeys it got really close there. But I made a great up and down (for birdie) on 15 and had a really good two-putt (for par) on 16 and on 18 that fairway bunker shot was probably the best shot I hit all week."

Johnson had several putts lip out in the first round but while he was not at his best on the greens, he was still more clinical with the blade than McIlroy, never looked inspired and went out in one over par to find himself six behind Johnson.

The Holywood star stole the FedEx Cup from under Johnson's nose last year but while he picked up a shot coming home, he had 30 putts in a level par 71 that was only good enough for a share of seventh on 10 under par.

The good news is that his injured ribs held up well and he now has two more events to get his razor sharp for the Masters and his latest bid to secure the elusive final leg of the career Grand Slam and a place in golfing immortality.

Ranked second from tee to green and fourth for driving distance, McIlroy was 24th out of 76 for putting with two shot missed putts on Friday evening leaving him just two shots ahead of the field entering the weekend when he might have lead by four or five.

"The course got a little trickier over the weekend," McIlroy said. "Maybe I just didn't quite adjust to that. But all in all, first week back, it's okay.  But being two ahead going into the weekend, obviously disappointed with the finish I had.”

Fleetwood holed from nearly 40 feet on the last to complete a superb 66 and secure the biggest payday of his career of over $1 million and open up a gap of 965,676 points over Fabrizio Zanotti at the head of the Race to Dubai Rankings Presented by Rolex.

Johnson only moved to the top of the Official World Golf Ranking with victory in the PGA Tour’s Genesis Open a fortnight ago but he has now established himself as the man to catch at the top of golf’s world order. 

Fleetwood's performance will take the 26-year-old to a career-high of 35th in the world, with Rahm up to 25th.

“It's tough to beat the best player in the world, isn't it?" Fleetwood said. "It was a great back nine.  I think started pretty well after all a couple bad shots. So I felt comfortable on the back nine and made the most of those two par 5s and a couple more birdies at the last. 

“I wasn't overly worried about where the leaders were. Dustin and Jon kind of came backwards a little bit. I didn't know what Jon was doing, I don't know if he made a couple bogeys or double bogey or something. 

"To be honest, I was kind of looking at trying to get third on my own at one stage, and then before you know it I saw the scores on the last green and I thought I might have a chance at winning it.  But it was a really good putt on the last.  It was a big one.  It would have been nice to get in a playoff and see how we did, but overall it was a good day.”

Rahm was only thinking of the positives, despite those late errors.

He reached 14-under and held the solo lead for a short time after beginning the back nine par-eagle-par-par-birdie-birdie,

One of six players making their World Golf Championships debut, he missed a seven-foot par putt at the 16th and an eight-footer at the 17th to fall two strokes behind Johnson. 

The 22-year-old Spaniard was No. 137 in the Official World Ranking before winning the Farmers Insurance Open to advance to No. 46. 

Two more strong performances (T16/Waste Management Phoenix Open, T5/AT&T Pebble Beach Pro-Am) moved him to No. 37 helping secure his position inside the top 50 and earn a spot in the field at the Mexico Championship.

“You know, I didn't play great every day and still had a chance to win the tournament," he said. "That's exactly where — we're not going to play great every single day, but the days that we don't have our A game, to have respectable scores is going to keep you going for a long time and I was just glad I could do that.  

"My first chance here on this golf course, first time I played this kind of event and to actually have a chance and be in it the first time, it's really nothing but positive."

Johnson's 14th PGA Tour win means he becomes the fifth player, and first since Adam Scott in 2014, to win his first PGA Tour start as the No. 1 player in the Official World Golf Ranking.

World Golf Championships-Mexico Championship    

Final-Round Leaderboard

1 Dustin Johnson 70-66-66-68—270 (-14)

2 Tommy Fleetwood 69-70-66-66—271 (-13)

T3 Ross Fisher  67-68-72-65—272 (-12)

T3 Jon Rahm 67-70-67-68—272 (-12)

T5 Thomas Pieters 68-69-68-68—273 (-11)

T5 Justin Thomas 69-66-66-72—273 (-11)

Dustin Johnson’s 14 PGA TOUR victories:          

2008 – Turning Stone Resort Championship

2009 – AT&T Pebble Beach Pro-Am

2010 – AT&T Pebble Beach Pro-Am, BMW Championship

2011 – The Barclays

2012 – FedEx St. Jude Classic

2013 – Tournament of Champions

2014 – World Golf Championships-HSBC Champions

2015 – World Golf Championships-Cadillac Championship

2016 – U.S. Open, WGC-Bridgestone Invitational, BMW Championship

2017 – Genesis Open 

2017 – World Golf Championships-Mexico Championship

Johnson becomes the fifth player, and first since Adam Scott in 2014, to win his first PGA TOUR start as the No. 1 player in the Official World Golf Ranking.

1991 Ian Woosnam — Masters Tournament

1999 David Duval — BellSouth Classic

2004 Vijay Singh — RBC Canadian Open

2014 Adam Scott — DEAN & DELUCA Invitational

2017 Dustin Johnson — World Golf Championships-Mexico Championship

Johnson (14/2008-present) has at least one win in each of his first ten seasons on the PGA TOUR (the best current streak) and joins Arnold Palmer (17/1955-1971), Jack Nicklaus (17/1962-1978) and Tiger Woods (14/1996-2009) as the only players since 1960 to achieve this feat.

Johnson’s Statistics By Round

1. R1: 10/14 fairways, 13/18 greens, 30 putts
2. R2: 9/14 fairways, 13/18 greens, 27 putts
3. R3: 8/14 fairways, 14/18 greens, 27 putts
4. R4: 6/14 fairways, 12/18 greens, 27 putts

Johnson’s Strokes Gained Summary (week)

1. Off-the-Tee +1.130 (27th)
2. Approach-the-Green +8.729 (1st)
3. Around-the-Green  +2.730 (8th)
4. Putting +1.608 (21st)
5. Total   +14.197 (1st)