American Phil Mickelson was the early clubhouse leader after shooting a three-under-par 67, with only Nicolas Colsaerts of Belgium, who fired a 69, the only other player to better par from the early finishers.
Commentators and some leading players, including The Open champion Ernie Els, had predicted that birdies would fly at Merion, which has not hosted a Major in 32 years, amid concerns the modern game had outgrown it.
"I think that (people) in that commentary box have never given this golf course enough respect," said Englishman Ian Poulter, who started out Thursday morning with three birdies in a row after teeing off from the 11th before finishing with 71.
"They were joking around laughing at 63s and 62s and just look at the board. I mean they need to respect this golf course. It's brutal.
"The long holes are severely long. Yes, we're making birdies on those short holes, but look what we have to contend with out there on those long holes. It's not easy."
Rickie Fowler, 24, playing his fifth US Open, said the deep, punishing rough was proving to be a strong defence.
"There's some nasty rough out there," said Fowler, who also shot a one-over 70. "It is short on the card, and it was playing soft today, which makes it play longer.
"I actually think this course plays more like a 7,400 or 7,500 yard golf course...
"If you drive the ball well out here, you can attack it right now with the greens being receptive.
"But you're not able to play out of the rough. You're not able to get much more than maybe seven-iron out of the first cut off the fairway, and then once you get out a little deeper, you've got wedge in your hand."
Mickelson, seeking his first US Open crown after finishing runner-up a record five times, praised the US Golf Association set-up of the classic course where Bobby Jones capped his 1930 Grand Slam by winning the US Amateur.
"What I love about Merion and what they did to Merion in the set-up is they made the hard holes even harder," the left-hander said.
"They moved the tees back on the more difficult holes, which made it even tougher pars. I love that because if you're playing well, you're going to be able to make pars and you're going to be able to separate yourself from the field by making pars.
"But on the easy holes, they didn't trick them up and take away your birdie opportunities. They gave you birdie opportunities to get those strokes back.
"So I think it's the best US Open set-up I've ever seen."
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- Nicolas Colsaerts