Tiger Woods briefly seized the US Open lead early in his second round at The Olympic Club, but he was soon back in a pack that included 17-year-old US amateur Beau Hossler.
The former world No.1 on Friday took the tournament lead for the first time when he birdied the par-three third, but no sooner had he poked his nose in front than he went into reverse gear with three bogeys in a row from the fifth.
That left him at one over for the tournament, two strokes off the one-under clubhouse lead that was held by US veteran Jim Furyk, the only man in the morning groupings to better par at the halfway stage after a 69.
Also safely in the clubhouse, tucked away from the fangs of the feared Lake Course at one-over were US qualifier John Peterson (70), rising Belgian player Nicolas Colsaerts (69) and the 2010 US Open champion Graeme McDowell of Northern Ireland (72).
Out on the course, overnight leader Michael Thompson had steadied after his poor start and was holding firm at one under through 10 holes.
The only other player in red figures was the surprising Hossler, a Californian high-school student playing in his second US Open.
After an impressive opening par 70 on Thursday, he had 10 pars and one birdie to open his second round.
World number one Luke Donald and defending champion Rory McIlroy appeared destined to miss the cut, Donald with a two-round total of 151 and McIlroy on 150.
On the course, four-time major champion Phil Mickelson and Masters champion Bubba Watson were trying to rebound from disastrous opening rounds, hoping to do enough to make a halfway cut that was to incude the top 60 players and ties.