Rory McIlroy's U.S. Open defense was hanging by a thread after the Northern Irishman made a poor start to his second round at the Olympic Club on Friday.

Needing to shoot a low score to make the cut after opening the tournament with a seven-over-par 77 on Thursday, he slipped further down the leaderboard after two early bogeys.

Starting his round from the ninth hole, McIlroy dropped a stroke on the 451-yard 12th hole when he three putted after his birdie putt narrowly missed and rolled off the green.

He made another bogey on the par-three 13th, the only hole he birdied in the first round, but picked up a stroke with a birdie on the par-five 17th, leaving him eight-over for the tournament midway through his second round.

With nine holes to play, he was outside the top 100, 12 strokes behind overnight leader Michael Thompson, who was yet to tee off. Only the top 60 players, plus players tied for 60th, make the cut.

World number one Luke Donald, playing in the same group as McIlroy, was also facing an early exit after he dropped a shot to be 10-over with nine to play.

The Englishman made a birdie on his opening hole, his first of the tournament after disastrous 79 on Thursday, and a second birdie on the 14th but three bogeys saw him slide further away.

While McIlroy and Donald both struggled early on, Graeme McDowell, the 2010 U.S. Open champion, recovered from a sloppy start to reach two-under and put himself in contention for a second major.

The Northern Irishman bogeyed two of his first four holes but made three birdies, including two in a row, to leapfrog Tiger Woods and David Toms into outright second place.

Woods and Toms were scheduled to tee off in the afternoon after finishing the first round in a five-way tie with McDowell, Justin Rose and Nick Watney at one-under.

Rose made bogeys on the second and fifth holes to slip back to one-over while Watney, who made a rare albatross on the first day, plummeted down the leaderboard when he dropped six shots in his front nine, including a double-bogey on the second hole.

(Editing by Frank Pingue)