The former Masters champion appeared to have control of the year's second major as he safely plotted his way around the back nine at challenging Merion Golf Club before coming unstuck, like so many others, on the notoriously difficult closing stretch.
Schwartzel carded a one-under-par 69 to finish one shot behind pacesetting Phil Mickelson, but was happy with his score on a layout which has tested the game's best players to the full because of tough pin positions, sloping greens and heavy rough.
"Whenever you shoot an under par on Saturday at the U.S. Open, you can't be too disappointed," the 28-year-old South African told reporters after mixing four birdies with three bogeys on a hot, breezy afternoon at Merion.
"There are a few easy holes out there that you got to take advantage of, which I did. And then you've got the hard ones, which are where you come in and finish on.
"So you're going to get some and you're going to give some, so anything under par is fantastic here."
Schwartzel, who won his first major title by two shots at the 2011 Masters, rebounded from a bogey at the par-five second with birdies at the third, fourth and seventh to join Englishman Luke Donald in an early tie for the lead.
The slender South African was then handed the outright lead at one-under when Donald bogeyed the par-four sixth but the leaderboard kept fluctuating wildly as six different players had turns at the top over the first nine holes.
Schwartzel again moved one stroke clear, at two-under, when he birdied the driveable par-4 10th but he was unable to build on that before ending the day at level-par 210, along with Americans Hunter Mahan and Steve Stricker.
"It's going to be a tough day, but every day has felt like that," Schwartzel, an eight-times winner on the European Tour, said of his expectations for Sunday's final round. "You're going to have to give every single shot all your attention.
"There's no hole where you can sort of ease back and free-wheel a bit. Every shot that you hit is really intense. You've got to really stick in there tomorrow and try and hit the shots the best you can."
Asked to assess the difficulty of the final five holes on Merion's iconic East Course, he replied: "If you can pull through there in level par, if you're one or two off the lead, you would be, worst-case, in a playoff.
"But if you're tied or leading and you play those few in level, you would be pretty unlucky not to win."
- Sports & Recreation
- Merion Golf Club