Well, we never expected this at the start of the BMW PGA Championship.

But then, who ever said golf was predictable? Amid all the tumult and the scrutiny of his highly-publicised off-course issues, Rory McIlroy did what he does best on the course.

This was an astonishing victory on an astonishing day at Wentworth. Caroline Wozniacki had been left down in the dumps after McIlroy called off his wedding to the Danish tennis player just a few days ago and whipped up a salivating media feeding frenzy.

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Last night, he left another Dane feeling crushed. Thomas Bjorn had led by five strokes at the start of the day and was seven ahead of McIlroy with 18 holes to play but, while one player collapsed in agonising fashion, another soared majestically. The Northern Irishman's superbly assembled six-under 66 for a 14-under aggregate of 274 gave him his first victory of 2014 and a first ever professional triumph on European soil, while Bjorn slithered back into a tie for third with a torrid score of 75.

Shane Lowry, a close friend and former Irish amateur team-mate of McIlroy, sneaked in at the death to seize second on his own with a raking birdie putt on the 18th at the end of a bewildering, captivating afternoon of wildly fluctuating fortunes.

In what rapidly developed into a thrilling finale, Luke Donald, the two-time PGA champion, also had aspirations of glory but those ambitions ended with a second shot into the water on the 72nd hole when he was going for broke.

McIlroy may have won his two major titles by a whopping eight shots but, in the mind-mangling circumstances, this certainly has to rate as one of his finest triumphs. "It's been a great day," he said after becoming only the second player from Ireland, after Harry Bradshaw in 1958, to land the PGA prize.

"After all that's happened, being inside the ropes was a release. Call it serenity, a sanctuary, whatever. But playing golf is what I do best. It is mixed emotions and I look at that trophy and think, 'how the hell did it happen this week?' Perhaps it was meant to be."

It did not take long for affairs to unravel and any hopes Bjorn had of enjoying a victory procession were swiftly trampled over. It ended up more like a funeral march for the 43-year-old.

McIlroy had served notice of his intentions with an eagle putt on the fourth that teetered and then dropped and Lowry made his move with an eagle-birdie surge at the fourth and fifth. Bjorn, meanwhile, found himself in a desperate guddle on the sixth as he took two shots to gouge himself out of a bunker on his way to a crippling triple-bogey 7.

His playing partner, Donald, did not fare much better and racked up a ghastly 7 of his own. It looked more like The Herald sports desk's spring outing than the sharp end of the European Tour's flagship event. The glowering Bjorn continued to toil desperately as the chasing pack bounded up behind him.

McIlroy chipped in on the 10th, while the burly Lowry birdied the 10th, 11th and 12th to muscle his way into a handy lead. It was fleeting, though. A visit into the trees on 13 cost him a double-bogey but he showed tremendous resolve to birdie the next and keep his title aspirations alive.

McIlroy was alive and kicking on, though. He made a further gain at the 17th and took the lead for the first time on his own. When he splashed a superb shot from the bunker to within four feet on the last and trundled in the birdie putt, the roars from the galleries could have torn the hubcaps off all the parked up BMWs.

It was a day of drama that even some of the best scriptwriters would have struggled to dream up. There were plenty of people getting in on the act and Stephen Gallacher emerged with a purposeful thrust as he came rampaging up the leaderboard with a sizzling 66. The Scot's record here at Wentworth is modest, to say the least.

Apart from a fourth-place finish in 2010, Gallacher had only made two other cuts in 14 previous appearances in this neck of the woods. This was more like it, though. A birdie at the first was the catalyst for a profitable afternoon and the 39-year-old upped the ante on the back-nine with a run of four successive birdies from the 11th which gave him a nine-under aggregate of 279 and a share of fifth.

"I was just trying to post as low a number as I could, but I was a wee bit disappointed that I couldn't knock off another birdie on the last two holes," said Gallacher.

McIlroy's own trip up Route 66 a couple of hours later would lead him to the ultimate, if unexpected, glory.