Dublin - Paul Casey has fired a six-under par 66 on Friday to seize the halfway lead at the US PGA Tour's Memorial tournament as overnight leader Rory McIlroy endured a nightmare day.
England's Casey had six birdies and an eagle with two bogeys on the Muirfield Village course with his 12-under total of 132 giving him a three-stroke cushion over Masters champion Bubba Watson going into the weekend.
Watson fired a three-under 69 for a nine-under total of 135.
Fellow American Chris Kirk was a further stroke back in third after a 70 for 136, and Japan's Hideki Matsuyama (67) shared fourth place with American Martin Flores (68) on 137.
Casey played the four par-five holes in five-under, including an eagle at the 15th where he drained an 18-foot putt.
"I was ecstatic with the way I played," Casey said.
"There was a couple of mistakes in there, but that's Muirfield Village.
"It's a tricky golf course and you don't need to do a lot wrong to make a mistake."
Casey's lone US PGA title came at the 2009 Houston Open, but he has 13 other global triumphs, including last year's Irish Open.
He started the day three strokes behind McIlroy, sharing second with Watson and Kirk.
But he found himself atop the leaderboard after a birdie at his second hole, the par-five 11th, thanks to McIlroy's swift collapse.
Former world number one McIlroy followed up his first-round 63 with a six-over par 78.
McIlroy, who arrived at this $6.2 million tournament hosted by Jack Nicklaus off a win in the European PGA Championship at Wentworth, was undone by a string of three straight double bogeys.
He bogeyed his opening hole, the 10th.
After a birdie at 12 he had double bogeys at the 13th, 14th and 15th to lie six-over for the day after his first six holes.
McIlroy three-putted the 13th from 75 feet and was in the water at 14. He almost reached the par-five 15th in two, but double-hit a chip.
A bogey at 18 saw him make the turn in seven-over 43 -- and even a second nine that included two birdies and one bogey did little to help his cause.
His 78 left him on three-under 141, nine shots off Casey's lead.
McIlroy said his score had nothing to do with a sore knee, which didn't bother him on the course.
"Seemed like anything that could sort of go wrong did go wrong out there," McIlroy said.
McIlroy, whose victory at Wentworth on Sunday capped a tumultuous week in which he announced his split from tennis star fiancee Caroline Wozniacki, insisted the big number wasn't "disastrous".
"Going into the weekend, not exactly where I want to be, but could be worse," he said.
Leading scores after Friday's second round of the $6.2 million USPGA Tour's Memorial Tournament (USA unless noted, par-72):
132 - Paul Casey (ENG) 66-66
135 - Bubba Watson 66-69
136 - Chris Kirk 66-70
137 - Martin Flores 69-68, Hideki Matsuyama (JPN) 70-67
138 - Hunter Mahan 68-70, Ryan Moore 68-70, Thorbjorn Olesen (DEN) 71-67, Scott Langley 72-66
139 - Adam Scott (AUS) 69-70, Camilo Villegas (COL) 71-68, Marc Leishman (AUS) 71-68, Scott Brown 70-69, Brendon Todd 71-68, Gary Woodland 71-68, Robert Streb 72-67
140 - Luke Donald (ENG) 71-69, Jason Dufner 71-69, Ben Curtis 69-71, Nick Watney 69-71, Bill Haas 73-67, Justin Hicks 73-67, Billy Horschel 71-69