While Tiger Woods and Phil Mickelson ended a frustrating day without a single birdie between them, Casey ground out a 2-under 69 to set the pace with American Shaun Micheel and Zimbabwe’s Brendon de Jonge.
Englishman Casey, bidding for Europe’s first success at the US Open in 40 years, overcame a mediocre display of ball-striking with superb scrambling and excellent putting.
“I had low expectations coming in but I did a wonderful job of managing my way around this golf course,” the world number nine told reporters after totalling just 23 putts.
Micheel, the 2003 US PGA champion, needed only 22 putts to claw his way into contention while de Jonge holed out with a wedge to eagle the treacherous 14th on his Open debut.
Little-known Spaniard Rafael Cabrera-Bello, competing in his first major championship, carded a 70 to finish level with South Korean KJ Choi, Canadian Mike Weir, Britain’s Ian Poulter, German Alex Cejka and Japanese teenager Ryo Ishikawa.
Triple champion Tiger Woods, despite finding every green in regulation on the front nine, struggled with his putting en route to a 74, one stroke better than Mickelson.
Woods, who romped to a 15-stroke victory when the US Open was last held at this venue in 2000, totalled 34 putts while short game magician Mickelson needed 32.
“The greens are just awful,” world No. 1 Woods said after a round featuring bogeys at the ninth, 16th and 18th.
“You know they’re going to be bouncing all over the place and you can’t leave yourself a long second putt. You’d rather be 30 feet below the hole than 10 feet above the hole.”
US Masters champion Mickelson, who has finished runner-up five times at the U.S. Open without ever winning, was equally annoyed after missing five birdie putts from within 10 feet.
“I just putted horrific,” the American left-hander said. “It’s very frustrating for me to miss all those opportunities. I’m hitting the ball well. I’ve just got to get sharp on the greens.”
Although the early starters had to contend with only light breezes on the spectacular coastal layout, Pebble’s notoriously small greens posed continual problems on a course running fast and firm.
Several players—de Jonge, Choi and Weir among them — briefly got to three under before slipping back over the tricky closing stretch.
Although de Jonge faded to level par with five holes remaining, he vaulted back into contention with his unexpected eagle at the par-five 14th.
“It takes a lot of luck when something like that goes in,” the 29-year-old said. “That’s such a hard third shot, and such a hard hole, period.
“It got a little bit bumpy this afternoon on the greens and you have to try to stay patient. I’m pretty excited but I’m trying not to get too far ahead of myself.”
Micheel, who struck a brilliant seven-iron approach to two inches on the final hole to seal a shock victory at the 2003 PGA Championship, is playing this week in honour of his cancer-stricken mother Donna.
“She’s been having a lot of bad days,” he said after sinking several clutch putts on Pebble’s quickening greens. “I’m really playing for her, and it’s nice to actually play for someone other than me.”
Only nine players in the field of 156 carded sub-par rounds on a day when the average score was 75.28 and the course set-up was widely praised by the players.
For flamboyant Englishman Poulter, the likely winning score in the year’s second major was irrelevant after the opening round.
“I’m not thinking about what kind of score might win this golf tournament,” the world No. 8 said. “I’m just happy to go out there and play as good as I possibly can.
“But I will tell you the golf course is difficult and I can’t see it getting any easier.”
