Wolff, 21, began the day four shots behind overnight leader Patrick Reed but erased that deficit in seven holes en route to a five-under-par 65 that tied the low round of the week and brought him to five under overall.
“I like to go out there and do what I feel comfortable with, rip dog and see how it goes from there,” said Wolff, who will look to become the first player to win the U.S. Open in his tournament debut since Francis Ouimet in 1913.
“I feel comfortable with every part of my game so I don’t like to shy away from things when I’m feeling confident, and I’m probably going to do the same tomorrow.”
The front nine at Winged Foot is one of the more daunting stretches in golf but a steadfast Wolff came out firing lasers at tricky pin positions and reached the turn with a blistering five-under 30 after five birdies.
Wolff, who hit just two of 14 fairways, made his lone bogey at the par-four 16th but responded in style with a birdie at the last after sticking his approach shot to 10 feet.
It was another difficult day of scoring at Winged Foot as only seven players managed to break par on a layout renowned for its narrow fairways, thick rough and wildly undulating and speedy greens.
Among those who struggled under the tough conditions and faded from contention were former world number ones Justin Thomas and Jon Rahm after matching 76s. REED STUMBLES
DeChambeau (70), who began the day one shot back of Reed, made a bogey-bogey start but leaned on his power to chase down Wolff and squandered a chance to finish within one shot of the lead when he two-putted from six-feet for a closing bogey.
“Tomorrow I’ve just got to make sure my speed is a little bit better than it was starting out the round and hitting the driver in the fairway,” said DeChambeau, who finished in a share of fourth at last month’s PGA Championship.
“Today was pathetic and it needs to be better than that to win a U.S. Open.” Former British Open champion Louis Oosthuizen (68) was alone in third place, four shots back of Wolff, while Hideki Matsuyama (70), Xander Schauffele (70) and Harris English (72) were each a further shot adrift.
Four-times major champion Rory McIlroy was six shots back of the lead.
World number 10 Reed (77) drained a nine-foot birdie at the ninth to share the lead but fell apart over a nightmarish inward nine during which he made six bogeys and a double-bogey to fall into a share of 11th place, eight shots back of Wolff.
“Well, I got all my bad shots out of the way,” said former Masters champion Reed. “You know, it was just one of those days. I couldn’t find a fairway, and from there trying to guess out of the rough all day, it was just hard. It was brutal.” Reporting by Frank Pingue in Toronto; editing by Ian Chadband and Ken Ferris