The 29-year-old, despite not feeling at his best on the practice range, fired a four-under-par 66 to surge within five strokes of the early lead in the second round at Oakmont Country Club.
''That round ranks as one my finest ever,'' a beaming Casey told reporters after rattling up five birdies and a lone bogey on the par-four 18th.
''If I had kept the bogey off the card, without a doubt it would have been the best.
''This is possibly the toughest golf course I have ever played and I feel very, very lucky to have shot 66 on it.'' Conditions were brutal on the second day at a sun-baked Oakmont where the hilly, par-70 layout with its notoriously difficult greens ran faster and firmer in swirling breezes.
Englishman Casey, an eight-times winner on the European Tour, conceded he was a little lucky to return his best score in five U.S. Open appearances.
''I got a couple of fortunate breaks but the rest of it was good ball-striking,'' he said, after dropping his only shot at the 18th where he lipped out from six feet.
''I spent a lot of time on the range yesterday, which is rare for me. I left the range not particularly happy with the swing but it just clicked this morning, so I'm ecstatic.'' Casey, whose best major finish was a tie for sixth on his Masters debut in 2004, feels he has underperformed in the big events.
''I'd love to win a major but I think I've got in my way in the past and I probably got in my own way yesterday,'' he said, referring to his opening 77.
''I prepared thoroughly and maybe I put too much pressure on myself but I'm off and running now so I can enjoy the last three days.'' Reuters AKD VP0105