Rory McIlroy has admitted that he is at a loss to explain his dramatic loss in form after a hugely disappointing opening round at the 2013 Open Championship.

The world number 2 is carded a dismal 8-over par 79 and will now struggle to make the weekend cut.

At the top of the leaderboard is American Zach Johnson. His 5-under par 66 today has yet to be matched at Muirfield and is a shot clear of Rafel Cabrero Bello.

Padraig Harrington and Darren Clarke are both on level par.

Graeme McDowell was also on level par after his opening 4 holes, having teed-off alongside Tiger Woods and Louis Oosthuizen in the afternoon session.

Woods recovered from an opening bogey to return to level par.

Earlier, Shane Lowry finished his opening round on 3-over par.