Greens that had been cut three times and rolled twice, combined with warm sunshine that cooked the course, led to only two rounds under par and the highest weekday scoring in 21 years.
"I don't know what the average score was, but I think I shot under par,'' Woods said after a 74 that put him five shots back.
Even more shocking than the toughness of Oakmont was seeing Casey with a 66, a round so superb that players on the practice green who watched him finish on No. 9 applauded when he knocked in his final putt.
The average score Friday was 76.933, the highest before a cut at the U.S. Open since it was 77.8 in the wind-blown first round at Shinnecock Hills in 1986.
"I consider the U.S. Open to be the toughest test in golf,'' Casey said. "This is possibly the toughest golf course I've ever played, and I feel very, very lucky to have shot 66 on it. There is no rest out there.''
And there might not be any relief in sight.
The USGA said it would water the greens overnight, but with more sunshine in the forecast, Oakmont figures to get even more brutal.
Stephen Ames had the other subpar round (69), leaving him at 142 along with Aaron Baddeley (70), Justin Rose (71) and Niclas Fasth (71). Casey was at 143, with David Toms in the group another shot behind.
The USGA, as usual, offered no apologies.
"It's a hard golf course. We've said that all along,'' said Jim Hyler, head of the championship committee for the USGA.
Trevor Immelman leaned against a wall in the locker room after a 79 that eventually sent him home. He stared blankly at the television, watching other players suffer, trying to come to grips with how Casey posted five birdies and only one bogey.