Former Masters champion Zach Johnson began the 142nd Open Championship in the same imperious style as he did the 141st, but there was no let up in the misery for world number two Rory McIlroy.

While Ian Poulter labelled some of the pin positions at a parched Muirfield as "a joke", McIlroy blamed himself for a "brain dead" round of 79 which left him 13 shots off Johnson's lead.

Johnson carded a five-under 66 to lead by one from Spain's Rafael Cabrera-Bello and 1998 champion Mark O'Meara, with 1996 winner Tom Lehman a shot behind alongside fellow Americans Dustin Johnson and Brandt Snedeker, Spain's Miguel Angel Jimenez and India's Shiv Kapur. Kapur held the lead when he raced to the turn in 30, but dropped three shots on the back nine.

World number one Tiger Woods, seeking the 15th major title of his career and a first since 2008, was ominously poised on two under, but US Open champion Justin Rose's hopes of back-to-back majors suffered a blow with a 75 and defending champion Ernie Els was only one shot better off.

Former champion Darren Clarke was the best of the Irish after the opening round at Muirfield.

The 2011 Claret Jug winner posted a one-over-par 72 at the Scottish links on a day when the course got the better of most of the 156-strong field.

Clarke finished the day in a share of 27th place as he posted four birdies and five bogeys, which included dropped shots on the opening and closing holes.

Two-time Open winner Padraig Harrington is next best of the Irish contingent after posting two over par, one shot behind Clarke and seven shots behind the overnight leader Zach Johnson.

Shane Lowry is another shot back on three over, while Graeme McDowell had to settle for four over after dropping three shots in his final four holes.

Gareth Maybin also struggled, playing in the trickiest afternoon conditions, to finish on seven over.