Graeme McDowell's Wales Open defence was severely blown off course as Alex Noren took charge at Celtic Manor.
US Open champion McDowell went from seven under par overnight to one over in the opening seven holes, before finishing round three on three over.
Swede Noren leads by a shot on eight under, while fellow countryman Peter Hanson and Anders Hansen of Denmark are a shot behind.
Wales' Jamie Donaldson leads the home charge in Newport on six under.
A stiff wind and tough pin placings wreaked havoc in the Usk Valley and McDowell was one of the biggest casualties as the world number five suffered his worst round since the 2008 Open Championship at Royal Birkdale.
McDowell had hailed this Twenty Ten as one of his favourite courses before the £1.8m tournament but his opinion might have changed following his Saturday capitulation.
The Ulsterman's Welsh triumph last year was the catalyst to a stellar 2010 that culminated in a memorable Ryder Cup-clinching singles win over Hunter Mahan, as Europe beat the Americans here last October.
But after his first successive sub-70 opening rounds this year, McDowell endured the worst possible start with a double bogey six after finding two bunkers and suffering a three putt at the first.
McDowell, who fell from the lead at last month's Players Championship with a closing 79, dropped more shots at the second and fourth before going from the rough and into a ditch at the 433-yard fifth for another double bogey.
The 31-year-old, who will begin his US Open defence at Congressional on 16 June, hooked out of bounds on the par-three seventh before salvaging a double-bogey five with his second ball.
McDowell rallied with two birdies before the turn but a bogey at the 11th, where his second shot kicked into the water, followed by an eight at the par-four 12th - where two chips from off the green fell back at his feet - ensured he crashed out of contention of retaining his title.
And it could have gone from bad to worse for McDowell.
"I nearly had a two-shot penalty to add to my eventful day," he explained.
"When I saw the result that I didn't quite get to the green again on my second chip on 12, I clumsily sort of tapped any damage down and then looked up as the ball came rolling back down past me.
"My intent was not to improve my next potential lie. It was simply an instinctive reaction to the chip shot, in disgust."
Fortunately for McDowell, referee John Paramor agreed that his actions were in anger rather than to gain an advantage.
McDowell's day had been bad enough as he acknowledged: "I got off to a start where everything that could go wrong, did go wrong.
"It was one of the craziest seven holes I've played in a long time. My head was spinning. I felt I got heavily punished for some mediocre golf. It was a disappointing day but stuff happens."
McDowell's playing partner Noren, on the other hand, was the epitome of consistency with a solid par round of 71 amid the golfing carnage on the Twenty Ten course, described by television commentator Wayne Riley as "the torture chamber," where just 13 of the 64 players broke par.
Noren suffered three bogeys on the opening nine holes before three birdies in four holes between 11 and 14 steadied the 29-year-old, as he seeks his second European Tour victory.
Hansen took route 66 around the Celtic Manor course in the best round of the day, sealed with a stunning eagle at the 575-yard 18th where he was inches from an albatross two at the closing hole.
The two-time PGA champion recovered superbly from two bogeys in the opening three holes to move into contention for the £300,000 first prize.
Hanson, meanwhile, completed a Scandinavian feel to the top of the leaderboard with another steady display - despite bogeys in the opening two holes - to slip into the chasing pack ahead of Sunday's closing round.
Earlier on Saturday, US Open runner-up Gregory Havret withdrew from the Wales Open following the death of his father.
The 34-year-old was two over par after the opening two rounds at Celtic Manor and just made the cut, but pulled out at the beginning of day three.
Last year Havret almost became France's first major champion since 1907, as Graeme McDowell won at Pebble Beach.
Havret, whose last title was the 2008 Johnnie Walker Championship, is exempt from qualifying for the 2011 US Open.
The US major will be held at Congressional this year and starts on 16 June.