The English trio of Willett, Luke Donald and Ross Fisher made up the final three-ball and were the only golfers who had a realistic chance of claiming the title and the first-prize cheque of $1.25 million.
Donald began the day with a one-stroke lead over Willett, with Fisher another two shots back.
But Willett roared out of the blocks, his list of birdies in the first five holes earning him a comfortable lead, which he built on with another shot gained at the par-five 10th, taking him to 16-under through 12 holes.
Fisher, who won the Tshwane Open in Centurion earlier this year, was tied for second place on 12-under with Donald after going out in two-under 34 thanks to consecutive birdies at the eighth and ninth holes.
The tall 34-year-old then drained a 15-foot birdie putt on the 10th, but a wayward drive on the 11th, rebounding off the trees but not allowing him a shot at the green with his second, led to a bogey.
Donald also clawed a shot back when he birdied the par-five ninth after laying up – Willett getting on in two but misreading his eagle-putt – but the former world number one immediately gave the stroke back with a bogey at 10, a visit to a fairway bunker which he couldn’t initially get out of costing him an extra shot.
The fickle fortunes of golf were clearly displayed in the opening holes of the final round as Willett snatched a three-shot lead.
Overnight leader Donald must have been contemplating what a harsh mistress the game can be as, on his birthday, he bogeyed two of the first five holes and watched Willett fire his way to the top of the leaderboard with three birdies.
And just to top his frustration, Donald birdied the par-four sixth after a super approach shot but watched Willett escape from trouble as he salvaged a par with a brilliant up-and-down after being embedded in the bunker behind the green.
Willett had a perfect record from the fringe, a delightful chip down the hill on the par-five second setting up a birdie and the 27-year-old then rolled in birdie putts from the fringe on the third and fifth holes, both par-fours.
Donald’s 10-foot birdie putt on the sixth pulled one stroke back after bogeys on the third and fifth holes led to a pair of two-stroke swings with Willett.
On the third, Donald put his approach into the bunker left of the flag, short-siding himself, and an awkward lie meant he could only chip across the green. The birthday boy, celebrating his 37th, short-sided himself again on the fifth with a wayward approach shot over the green.
Fisher would have been perfectly happy with his start as a 10-foot putt on the par-three fourth gave him his second birdie. But successive bogeys then undid those gains, his approach on the fifth finishing short of the green and a wayward drive on six, followed by his third shot going over the green, led to another dropped shot, leaving him five behind Willett.
Frenchman Alexander Levy was also contemplating the vagaries of the game as he took 10 at the par-five ninth, twice hitting from the drop zone into the water surrounding the green. To his credit, he was still smiling as he trudged off the green and he birdied the 10th and 11th holes.