Heavy rain during the championship build-up had initially left the par-70 layout too soft, in Donald's opinion, considerably lengthening the list of potential winners of the year's second Major.
However, that changed dramatically on Friday as tugging breezes, tough pin positions, thick rough and firming conditions in bright afternoon sunshine combined to test the game's best players to the full.
"It was a shot or two harder," Donald said after surviving a rollercoaster two-over-par 72 that included six bogeys and four birdies.
"This is a tough course and it's obviously showing that you don't need a course to be ultra long to make it difficult.
"US Opens get harder as the week goes on and the pins today were a lot more tucked. They were tougher to get to. A few were on little hills or slopes. It's very difficult to make those putts when the ball is breaking so much."
On Tuesday, Donald rued the fact that the relatively short, 6,996-yard layout had become softened by more than five inches of rain over the previous four days.
"It makes the course a little bit easier," the English world number six said at the time. "It doesn't play quite as tough. As a top player you want the place to play as tough as it possibly can."
His prayers were answered on Friday, the course baring its teeth as the entire field struggled to shoot par.
"Merion is holding its own, for sure," said Donald, who shrugged off a run of five bogeys in six holes from the par-five second to post a level-par total of 140.
"I just feel like when you have wet conditions, the fairways become that much wider, the greens become wider, too. I just think it's going to help someone a little bit more because it's wet."
Asked what had mainly contributed to his stumbling run from the second hole, Donald replied: "Those were the par-fives, which I didn't play very well.
"The other bogeys I didn't feel like I made too many mistakes, it's just a difficult golf course, just a slight pull on five. But you try not to panic in US Opens; you try to take each hole as it comes.
"I made nine birdies over the last couple of days, and I've certainly played the par-threes very well. I think I made four or five twos out there, it's obviously been a big key to my score."
Donald, who has yet to claim his first Major title, likes his position going into the weekend as he contends for the Major widely regarded as the most gruelling to win.
"I would love to be a couple (of shots) better, but certainly I think come the end of round two, I'm going to be in a good place," said Donald, who stood three off the pace when he finished his round in the morning wave.
"I feel like I'm swinging pretty well and I've got a chance. So hopefully I can throw a good one in tomorrow and really be in the mix come Sunday."
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