Starting a stroke ahead of the field at Melbourne's Huntingdale Golf Club, the former world number one was five clear at the turn but came unstuck at the 10th, where a double-bogey stalled his momentum on a cool and drizzly afternoon.
From there it was a grind, as Scott dropped another two strokes and bogeyed the last to fall into a share of the lead with unheralded local Peter Wilson.
"It was a tale of two halves today," a deflated Scott told reporters after finishing with a one-under 70 for an eight-under total of 134.
"Had a lovely start, playing really solid and a couple of loose shots on the back nine and I was just grinding to get into the clubhouse actually.
"It's a shame. I felt like I didn't do too much, too bad but the driver kind of cost me three or four shots today. I just couldn't get it in the fairway and take advantage of the par-fives."
The world number 12, bidding for a third Masters title in four years, drained four birdies in the front nine and was bogey-free through 27 holes, but spent much of the day bashing sand off his shoes.
He came to grief with a nasty lie in a bunker on the par-four 10th to drop two shots, then bogeyed the 13th after whacking his tee-shot into water.
Another wayward drive found sand on the last hole and his putt to save par rolled well short.
Victorian professional Wilson shot a second consecutive 67 and relished the chance to be paired with Scott, the 2013 U.S. Masters winner.
"I've played with most of the guys now," Wilson said in a greenside interview, dropping the names of major champions Graham McDowell and Vijay Singh.
"They are just human to me. I don't get intimidated by anybody.
"(Scott) is up there with the (Greg) Normans and Tigers (Woods) of the world. Who knows what's going to happen tomorrow. I could shoot 64 and he could shoot 74, or vice versa."
Scott and Wilson were a stroke ahead of Queenslander Matthew Guyatt (66) in outright third.
American George McNeill, a two-time winner on the PGA Tour, was two shots adrift after firing a 66, with U.S. Amateur champion Bryce Dechambeau a further three off the pace on three-under.
(Editing by John O'Brien)
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