No unfinished business. Under dark, threatening skies at Valhalla Golf Club, Rory McIlroy – who has become golf’s ultimate closer of deals – secured back-to-back Major titles when adding the US PGA Championship’s Wanamaker Trophy to the Claret Jug he secured so impressively last month at Hoylake.

All the talk of this new era with a new king envisioning a lengthy reign ahead of him was given further fuel, as the 25-year-old Northern Irishman defied fading light and a posse of challengers to impressively claim a fourth career Major. A closing round 68 for 16-under-par 268 gave McIlroy a one-stroke win over Phil Mickelson, with Rickie Fowler and Henrik Stenson a shot further back in a share of third.

Unlike his three previous Major victories, in the 2011 US Open, the 2012 US PGA and the 2014 British Open, which were each secured with a confident strut and displays of brilliance, this time McIlroy had to show grit and resilience to match his majesty with the clubs. In darkness, he completed the task with a far from straightaway par on the 18th, punched the air in delight and embraced his caddie JP Fitzgerald.

In its own way, this final Major of the season proved to be the most captivating of them all. Bubba Watson was a clear winner of the Masters: Martin Kaymer turned in a masterclass in capturing the US Open: And McIlroy was a wire-to-wire winner of the British Open.

Here, on a course softened by a short but intense downpour which dumped over an inch of rain onto an already sodden surface and which led to a suspension of an hour and 45 minutes, McIlroy – who had carried a one-stroke lead into the final round – was caught by a succession of chasers and turned into the chaser on the home run.

But he proved to be very much up to the challenge. And as roars from ahead told him that Mickelson and Fowler and Stenson were lighting up the course with birdies, McIlroy – who reached the turn two shots adrift of the leading trio – found the supreme answer with a shot to the par-5 10th that proved to be a championship-defining moment.

There, after a superb drive down the middle of the fairway, McIlroy took a 3-wood from Fitzgerald and, from 280 yards, hit a stunning low approach shot that finished eight feet from the flag. He duly rolled in the eagle putt and, in that instance, all of the struggles of the front nine were wiped away.

In advance, McIlroy had remarked of gaining an understanding of the pressures which Tiger Woods had lived with for years. "You appreciate it even more when you get into this position what Tiger's done and the run he's gone on," admitted McIlroy.

McIlroy, though, proved that he too can deal with whatever pressures come with winning these biggest championships of all. In winning, he extended the streak of successes to three – the British Open, the WGC-Bridgestone Invitational and now the US PGA – and sent out a message, if it were needed, that he has indeed put his hand up to be the game’s dominant player.

This was a final round of many thrills. Fowler chipping in for birdie on the fifth, and later rescuing a par from the wrong fairway on the 16th; Stenson’s putting masterclass on the front nine in claiming five birdies on that stretch to power into contention: Mickelson majestically saving par on the 17th and very nearly chipping in for eagle the 18th.

But when all was said and done, it was McIlroy who was the last man standing. Although rushing to play the 18th to finish before darkness stopped play, McIlroy – who had a two-stroke lead on the 18th tee following a superb birdie on the 17th – narrowly avoided the hazard down the right and then found a greenside bunker with his approach. The recovery shot was followed by two careful putts for par, sufficient to secure victory.

There was almost embarrassment for US PGA president Ted Bishop as he dropped the lid of the trophy in the process of presenting it.

McIlroy, perhaps inevitably, made a one-handed catch to save Bishop’s blushes.

He’s that good right now.