The veteran South African had briefly got to within a stroke of the championship lead but struggled over Merion's notoriously difficult five-hole closing stretch, making three successive bogeys from the 14th.
"It was really blowing a bit this morning, probably a two-club wind," said Els, a two-times winner of both the US and Opens Championships.
"And it's dead into you on 14, 15, 17, then 18 is downwind. And it was raining a bit.
"So it was really more British Open championship stuff I should say. It was really difficult. You throw the rough in there, then the length of some of the holes, it's a tough finish."
Els had 10 holes remaining when he resumed the opening round on Friday morning and made a bright start with birdies at the ninth and 10th to get to two under, one behind Phil Mickelson's overnight lead.
However, his troubles began the moment he reached the par-four 14th as he missed the fairway off the tee and was able to advance his ball only a further 20 yards out of Merion's heavy rough.
The 43-year-old former world number one, popularly known as the 'Big Easy' because of his relaxed demeanor and imposing physique, also bogeyed 15 and 16 to drop back to one over.
"(On) 16 I only had a nine-iron and on 15, I kind of mis-clubbed myself, so those two bogeys sting a little bit," said Els, who clinched his fourth Major title at last year's British Open.
"It would have been great to get it into the red (under par). But I got to be fortunate, and I am happy with one over. Seventeen and 18 are just virtually impossible.
"It's very difficult and I was actually fortunate a couple of times. I didn't hit too many fairways, but I did get reasonable lies."
US Open champion in 1994 and 1997, Els ended the opening round with three birdies and four bogeys on his card, four shots behind leader Mickelson and tied for 16th place in a field of 156.
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