Casey shot a 4-under 66, by two shots the lowest round so far, to close within five shots of the lead after beginning his round in danger of missing the cut. Fellow Englishman Nick Dougherty, who held the first-round lead with a 2-under 68, was beginning his second round about the time Casey was ending his.
'They probably think I walked off after 14 (holes),' Casey said, laughing, when asked what the rest of the field thought of his score. 'I'm still a little bit stunned by it.'
Especially when there were more scores in the 80s than in half a year's worth of PGA TOUR events.
Phil Mickelson displayed the exasperation many felt as the sun dried out Oakmont Country Club and made it play up to its nasty reputation as a course that only grudgingly gives up birdies.
'Carnage,' Mickelson called it.
He looked to be making a move with two early birdies, only to take a tour of every trouble spot Oakmont can offer during a 7-over 77 that followed his first-round 74 and left him doubting he would make the cut for the first time in 31 majors.
Casey began his round on No. 10, so a dozen or so golfers were warming up on the adjacent practice green as he finished up. Those on the green and their caddies gave him an ovation, and one flashed -- in Casey's words -- 'a how-on-earth-did-you-do-that look.'
Casey could only shrug his shoulders in response.
A comeback second round is becoming a habit for Casey. At the 2006 U.S. Open at Winged Foot, he looked to be out of contention following a 77, but his 54-hole closing stretch was the tournament's best (72-72-69) and he tied for 12th. At the Masters this year, he followed a 79 with 68-77-71 and tied for 10th.
'I probably get in my own way sometimes,' he said, explaining he spent too much time Thursday worrying about his swing instead of simply going on the course and playing.
He also appreciated the support of a large gallery, one that grew larger and more supportive with every hole.
Casey likely wouldn't have been greeted so enthusiastically three years ago when, after the 2004 Ryder Cup in suburban Detroit, he was quoted as saying, 'Americans are stupid. I hate them' -- a reference mostly to how little they keep up on world affairs.
Curiously, Casey was living in Arizona at the time, and the remarks caused him so many problems for a while that he briefly sought professional help to deal with them.
Mickelson, looking much more confident and comfortable a day after laboring his way to 4 over, tried making a push as the second round began Friday. Instead, he fell apart during a four-hole stretch in which he went double bogey-bogey-bogey-double bogey, beginning with the difficult par-4 No. 7.
That mysterious Mickelson who frequently surfaces during majors -- the one capable of making a big move in a moment or falling out of contention with a sudden thud -- was back again.
As quickly as he picked up two shots to par, closing within four shots of Dougherty, Mickelson gave them back and more. In an instant, he brought back memories of his final-hole collapse at Winged Foot a year ago, when he gave away his one-shot lead and the U.S. Open with one terrible hole.
Again, the subplot of a U.S. Open became the misadventures of Mickelson. Perhaps he got a little too daring after waking up Friday with much less discomfort in his inflamed left wrist.
The wrist felt more like a bruise when touched, instead of the radiating pain he felt the past week, according to his trainer, Jim Weathers.
'Now it's bearable,' Weathers said.
Now if only his golf game was.
Mickelson wasn't the only one taking a second-round tumble. With sunny skies hardening Oakmont's fast-again greens, scoring conditions weren't as favorable as they were Thursday morning, when Dougherty and Angel Cabrera shot the day's only scores in the 60s.
By midday, there already were 20 scores of 78 or higher, including a 15-over 85 by former PGA champion Rich Beem.
Jim Furyk, considered a title contender, had a 75. Vijay Singh (77), Fred Funk (78), J.J. Henry (78) and Sean O'Hair (80) also had terrible days after being in the low 70s Thursday.
Still, the golfers should have been forewarned. The USGA issued this thinly veiled warning before play started Friday: 'Greens are back to their practice round speeds after a day of slightly slower speeds due to rain.'
In other words, good luck. If only Dougherty and Cabrera were in the 60s at Oakmont with the greens softer than usual, the temperatures moderate and a cooling breeze blowing through, what will the scores be by Saturday? By Sunday?
'We are in for a long week,' Singh said.