When the 34-year-old is in such form he is unstoppable. He showed this in abundance with his five-shot win at the World Golf Championships event in Mexico on Sunday. It was his second win of the year having triumphed in Saudi Arabia earlier this month.

But how different might it have been had he genuinely been pushed in the closing stages? Which brings us to his closest rival on the last day, the enigmatic Rory McIlroy.

And the bigger questions should surround the Northern Irishman after he collected a fourth successive top-five finish to start his year on the PGA Tour.

McIlroy undoubtedly had a chance to overhaul Johnson's four-stroke 54-hole lead but he lacked the ruthlessness to create the opportunity.

It will be a full year winless for the four-time major champion when he mounts his title defence of the Arnold Palmer Invitational at Bay Hill next week.

But it would be wrong to view this latest near-miss in a totally negative prism. Context is important and Johnson was superb; unbeatable given such distance off the tee combined with an unerring putter in his hands.

The American also benefitted from a tight, but correct, referee's call that gave him relief from the foot of a tree on the fifth hole. McIlroy was denied a similar break one hole later and the official, Mike Stewart, got that call right as well.

And then McIlroy, who had closed to within two shots after three holes, messed up with his third shot on that par-five sixth. He found the water to make a ruinous bogey.

Johnson birdied and was back in control heading to the seventh tee. There might have been a two-stroke swing at the ninth, but a combination of the chaser's hesitancy and the leader's composure ensured it did not happen.

McIlroy misread a decent birdie chance that would have applied pressure and Johnson holed a slippery par save. Then at the 10th the difference between the two players was further highlighted.

Johnson bombed a massive drive, only for his opponent to knock his past him. Crucially, though, DJ then finessed a delightful 61-yard pitch to tap-in range while McIlroy came up short of the green with his wedge.

It was game over, even though McIlroy then reeled off six birdies in the next seven holes. The champion eased home because the gap on the leaderboard was too big.

That one-over-par front nine was fatal and it was another tournament that got away from the 29-year-old Ulsterman.

It adds to a lengthy list of recent final-day frustrations but worries should be tempered because there is too much about McIlroy's current game that provides genuine encouragement.

Foremost, his putting stroke looks increasingly solid, especially on those devilish poa annua greens that destroyed, among others, Tiger Woods' efforts with the short stick last week.

McIlroy's ball-striking remains imperious and his wedge play, while not at Johnson's clinical level, is more dialled in. The European Ryder Cup star is 56 under par for his four events this year.

Few players would swap such a start but the likes of Justin Rose (Torrey Pines), JB Holmes, (Riviera) Xander Schauffele (Tournament of Champions) and now Johnson certainly would because of the way they closed out the kind of wins McIlroy craves.

He remains an intriguing character who never ceases to make headlines. His golf is never humdrum and his opinions forever spark debate.

Rory McIlroy is sixth in the world rankings

McIlroy was vilified by one newspaper columnist last week for turning his back on this year's Irish Open. It was "an insult to national intelligence", according to the Irish Independent.

Such interpretation was heavy-handedly ill-informed. All sorts of nuances are at play when schedules are considered and McIlroy has been a staunch supporter of his country's Open in recent years.

He will have been disappointed that the Irish tournament was not allowed to swap dates with its Scottish counterpart to be placed in the prime slot a week before The Open.

But the European Tour was in no position to frustrate Scottish sponsors who pay handsomely for that July date. McIlroy, meanwhile, wants to give himself his best shot of winning The Open, especially when it is returning to Northern Ireland.

So his Royal Portrush warm-up will take place in Scotland and not a week earlier at Lahinch.

If he had explained that fully at the first opportunity instead of calling the European Tour "a stepping stone" it might have saved him some media grief.

But the PR side of the game takes a secondary role. McIlroy prefers to speak his mind, which is one of the reasons he is such an interesting player.

The primary one, though, is his largely unrivalled ability to strike a golf ball. Only the irresistible force of an in-form Johnson could beat him in Mexico.

We know Johnson's form can fluctuate. McIlroy falls into a similar category, but the pattern of trend is encouraging and his timing might prove spot on.

There are big events ahead; a title defence at Bay Hill, the Players Championship back to its traditional March date and, of course, the Masters in April.

If it were all to click into place at Augusta, the frustrations and controversies of the past year would be quickly forgotten. It is a big but not insurmountable "if" and his current form does offer cause for optimism.