Kevin Na swings into early lead

The US Open lived up to its reputation as the toughest championship of all in the early going on Thursday with only five players under par in the opening round.

On a cool hazy day at the Olympic Club, American Kevin Na was alone at two under after six holes, a stroke in front of South African Branden Grace and Americans Jason Bohn, Joe Ogilvie and Davis Love III.

Grace, a three-times winner on the European Tour this year, had completed six holes and Bohn five while Ogilvie and US Ryder Cup captain Love had 14 holes to play.

Three-times US Open champion Tiger Woods was among a large group locked at even par, having parred his first five holes after teeing off at the par-four ninth.  Seeking his first major title in four years, Woods was playing in a glamour grouping with fellow Americans Phil Mickelson and Masters champion Bubba Watson who were both struggling at two over.

Watson bogeyed the ninth and 11th to lie four strokes off the early pace on a par-70 Lake Course layout boasting arguably the toughest stretch of holes in major golf from the first to the sixth.

Defending champion Rory McIlroy of Northern Ireland was among the day's late starters, with world number one Luke Donald and third-ranked Lee Westwood.

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