Sunshine replaced rain at the US Open at Merion on Friday, turning the classic course into a nightmare for players as Phil Mickelson hung on to a one-shot lead midway through his second round.
Billy Horschel, winner of the Zurich Classic in April, was the exception, surging into contention with 67 for second place at one-under 139 on a day when sunny skies and lively breezes began to firm up conditions at the weather-delayed championship.
Tournament favourite Tiger Woods was five shots off the pace at three-over 143 along with world number two Rory McIlroy of Northern Ireland, both having carded 70s.
Mickelson led by two over long-hitting Nicolas Colsaerts when he started the second round in the afternoon wave, but two holes later they were tied after the left-hander bogeyed the first and the Belgian birdied the second.
Four-times major winner Mickelson proceeded to par the next eight holes, missing a short birdie putt at the eighth, while Colsaerts registered three bogeys and a birdie over the rest of his front nine to dip to level par.
Also at level par, were England's Luke Donald, at 140 after a 72, Steve Stricker (through nine), England's Justin Rose (seven) and 2011 Masters winner Charl Schwartzel of South Africa (seven), who overcame a double bogey at the first.
With fairways and greens firming up, bringing the rough even more into play, and the pins in tricky positions, second-round scores went soaring after players returned to a wet, chilly Merion early Friday morning to finish their first rounds.
When the weather-suspended first round resumed on Friday, overnight clubhouse leader Mickelson was one shot behind former world number one Donald, who was four under through 13 holes.
The Englishman bogeyed two of his last three holes to finish with a first-round 68, one shot behind Mickelson, who had a late tee off.
In the second round, Donald temporarily regained the lead with back-to-back birdies from the 12th, his second hole of the round.
But the world number six tumbled down the leaderboard with a string of four bogeys from the fourth before adding another birdie at the ninth to get back to level par for the tournament.
Woods battled through the course challenges and a left arm injury to finish with a 73 in the morning then an admirable 70 in the afternoon to reach the halfway stage at three-over.
"It's hard with the wind and the pin locations. They're really tough," said Woods. "A lot of guys are missing putts and blowing them by the holes."
The American's playing partner, Northern Ireland's Rory McIlroy, shot identical scores to Woods while Australia's Masters champion Adam Scott struggled with a second-round 75 for seven-over 147.
The rough, nourished by a week of rain, caused havoc for a bunch of former U.S. Open champions, including Angel Cabrera of Argentina, who shot an 81, American Jim Furyk (79) and Northern Ireland's Graeme McDowell (77).
"It's that hard, it's that difficult, it's that long," said McDowell, the 2010 champion and runner-up last year, who had a 36-hole total of 13-over 153.
"I'm disappointed, of course. It's not the way I wanted to play the last couple of days. But this place is very hard."
The conditions made low scoring more difficult as the day went on with bogeys outnumbering birdies 481 to 191 late in the round.
There were also 91 double-bogeys and 19 worse scores with players averaging rounds of nearly five-over-par 75.
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