Most of the damage was done on a disastrous opening nine where three consecutive double bogeys saw him go out in 43 at Muirfield Village.

Starting on the back nine, McIlroy bogeyed his opening hole, the par-four 10th, before parring 11 and picking the shot back up on 12.

It went rapidly downhill after that, though, as double bogeys followed on 13, 14 and 15 before two pars and a further bogey saw him reach the turn four shots off the lead.

That represented a 12-shot collapse on the back nine from yesterday where he had picked up two eagles, three birdies and a double bogey to record a 31.

McIlroy did manage to turn things round a bit on the back nine though, picking up birdies on the second and seventh while only dropping the one shot in between as he ended the day on three under par overall.

Despite that partial recovery though, McIlroy found himself seven shots behind Paul Casey midway through the second day as the Englishman roared clear at the summit.

Casey shot a 66 on Thursday to sit in the three-man group immediately behind McIlroy alongside Americans Bubba Watson and Chris Kirk, and he picked up where he left off with an eagle and two birdies in his opening six holes on Friday.

That left the 36-year-old on 10 under for the tournament and with a three-shot lead over the rest of the field.

Watson was his nearest challenger, the American one under after two on Friday, while Kirk was one shot further back having parred his opening two holes.

Dane Thorbjorn Olesen was the clubhouse leader on six under after shooting a 67 that included seven birdies and two bogeys.