"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.

"The course got a little trickier over the weekend. 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.”

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.

In contrast, Greystones' Pual Dunne ranked third for putting as he finished a career-best sixth behind Dean Burmester, who won the Tshwane Open by three shots on 18-under-par from Spaniard Jorge Campillo and Finn Mikko Korhonen.

Dunne, 24, looked certain to miss the cut when he bogeyed five of his first seven holes in Thursday's first round.

But he salvaged a two-over 73, then shot a 68 to make the cut by one before blasting rounds of 65 and 66 over the weekend to win €35,772 and take his earnings from his first eight starts of the season to €126,201.

After the start really happy with the weeks work thanks for all the messages. Congrats @BurmyGolf long time coming. Delighted for you. #gent

— Paul Dunne (@dunners11) March 5, 2017