Paul Casey turned in a stunning performance in the second round of the US Open at Oakmont, but Nick Dougherty was battling to avoid a nightmare.

Casey shined on Friday, shooting a four-under-par 66, the low round of the tournament, to vault into contention at the season's second major.

The USGA announced on Friday morning that the greens were back up to their practice round speed after Wednesday's thunderstorm, but that mattered very little to Casey but quite a bit to the rest of the field.

After digging himself a hole on Thursday, Casey dug in on Friday, firing five birdies with just one bogey to lie at three over 143 after 36 holes.

''A 66 is way beyond my expectations and I am still a bit surprised at the score I shot today,'' Casey said. ''This is right up there in terms of my best round ever. If I had kept the bogey off the card, without a doubt it would have been.

''But I consider the US Open to be the toughest test in golf. This is possibly the toughest golf course I have played and I feel very, very lucky to have shot 66 on it.''

Dougherty, who led after the first round on two under, was two clear of the field after a birdie on the 13th, his fourth hole, but immediately ran into trouble at the next.

A three-putt bogey there was followed by a double bogey six on the next, and another dropped shot on the 18th took him out in 38.

At one over for the tournament Dougherty was two behind Angel Cabrera of Argentina, who was level for his round and one under for the tournament, the only player in the field in red figures.

Starting his round on the 10th after a 77 on Thursday, Casey got off to a flying start with a birdie and kept up the momentum throughout his front nine.

Playing in his fifth US Open, the 29-year-old added birdies at the long par-five 12th, 14th and 17th. His lone mistake came at the 18th where his approach was off to the right and long, settling in the rough just off the green. He chipped to about 10 feet but two-putted for his first bogey of the day.

''I don't know what the difference was, yesterday was disappointing,'' Casey said of his 11-shot swing.

''I think I got maybe a little wrapped up in technique, in trying to swing the club correctly rather than hitting golf shots. It's only my own fault.''

Phil Mickelson looked like he was going to join Casey in carding a low round on Friday but he undid any momentum he built with birdies at the fourth and sixth with a horrible close to his front nine.

The left-hander double-bogeyed the seventh and added bogeys at the eighth and ninth Things got no better for him on the back nine as he started his inward half with a double bogey at the 10th.

A pair of pars were just temporary relief as Mickelson bogeyed 13, 15 and 16 and 18, sprinkling in a birdie at the 14th to lie at an out-of-sorts 11 over for the championship.

Tiger Woods was in a large group at one over to start the second round and dropped a shot through seven holes with two bogeys and a birdie.

Justin Rose and Aaron Baddeley had set the clubhouse target on two over.

Rose shot his second 71 of the tournament while Baddeley signed for a 70 on Friday.