Fox made a sizzling start to the year's first major on Friday morning (NZT), powering to a two-stroke lead after his first nine holes in Augusta following a two-hour rain delay.
However, two dropped shots on his back nine saw the 37-year-old close on three-under 69, leaving him in a share of sixth.
Fox's exceptional start featured birdies on each of the first three holes, followed by a brilliant eagle on the par-five eighth hole to go to five-under.
However, he didn't pick up another shot for the reminder of his round, shooting bogeys on the par-five 13th hole and the par-three 16th.
Fox was four behind American big-hitter DeChambeau, who was one stroke ahead of compatriot Scottie Scheffler, the world No 1 and tournament favourite who claimed the famed Green Jacket as champion in 2022.
Denmark's Nicolai Hojgaard was all alone in third on five-under with three holes still to play, sitting one shot in front of another former Masters winner, Englishman Danny Willett, and American Max Homa.
LIV Tour player DeChambeau was the standout performer, carding eight birdies, including five in the space of six holes on his back nine.
The first round nevertheless represents a remarkable reversal of form for Fox, whose world ranking has dropped to 54th in recent weeks following a string of modest returns on the US PGA Tour.
After a storm delayed the start of the year's first major, 2020 US Open champion DeChambeau thrived in the softened but windy conditions and mixed eight birdies with one bogey for a seven-under-par 65.
DeChambeau, who raised eyebrows in 2020 when he said the par-72 Augusta National layout for him played more like a par-67, enjoyed a fast start as he opened with three consecutive birdie putts, none longer than six feet.
The only blemish on DeChambeau's card came at the par-four ninth where he three-putted from 70 feet but he went on to scorch the back nine with five birdies over a hot six-hole stretch that he capped with a 31-foot birdie putt at the 17th.
"I have a level of respect for this golf course that's a little bit different than a couple years ago, and clearly today was a great test of golf, and I was able to conquer a very difficult golf course," said DeChambeau, who is one of 13 LIV Golf players in the field.
"Regarding the 67 comment, you know, you mess up. I'm not a perfect person. Everybody messes up. You learn from your mistakes, and that was definitely one."
World number one and pre-tournament favourite Scheffler, playing in a high-profile group with Grand Slam-seeking Rory McIlroy and Olympic champion Xander Schauffele, also rode a late birdie blitz to card a bogey-free six-under-par 66.
Scheffler, the 2022 Masters champion, was two under through 11 holes but caught fire with four birdies over his next five to pull within a shot of DeChambeau.
McIlroy, who missed the cut in last year's Masters and is making his 10th attempt at completing a career Grand Slam of golf's four majors, opened with a 71, while world number five Schauffele carded a 72.
Completely unexpected
Sitting in the clubhouse alone in third place was 2016 Masters champion Willett, who is making his first start since undergoing shoulder surgery in September and only decided on Sunday that he would play this week.
The 36-year-old Englishman reached the turn at three under but two bogeys over his next five holes knocked him out of contention until he birdied three of the final four holes.
"I think I might take the next six months off," Willett joked. "No, it's completely unexpected. Sometimes that happens, whatever. You make a couple of birdies and your mind starts thinking, all right, I can do it."
LIV Golf's Jon Rahm, bidding to become the fourth golfer to successfully defend his Masters crown, launched his title defence with a one-over-par 73.
Rahm reached the turn riding high after two consecutive birdies moved him to two under but he endured a shaky back nine that included four bogeys.
The delayed start to the first round means the day's late starters, including five-times champion Tiger Woods, 2023 runner-up Brooks Koepka and former winner Dustin Johnson, didn't finish their first rounds.