Fuelled by a blistering front nine that included two eagles, Aberg fired a superb nine-under-par 63 on a sun-splashed afternoon at TPC Sawgrass for a 12-under total.
The 26-year-old, whose most recent PGA Tour victory came at last year's Genesis Invitational, leads two-time major champion Xander Schauffele, who had set the pace in the morning wave with a seven-under 65.
Eight shots off the pace is Day, who managed a second consecutive round of 70 to sit at four under.
The Queenslander, victor in the PGA Tour's flagship event in 2017, didn't have a bogey on his flawless card on Friday and his only dropped shots after two rounds came in Thursday's first round on the course's signature par-three 17th.
Aberg made an electrifying start to the second round. He birdied the first, eagled the par-five second by sinking a 4m putt and picked up further shots at the third and fourth.
He then added another eagle at the par-five ninth, where he chipped in from light rough just short of the green, to reach the turn at a sensational seven under and in a tie for the lead with Schauffele.
Aberg briefly moved one ahead with a two-putt birdie at the par-five 11th before slipping back with a bogey at the par-four 15th. Undaunted though, he responded with additional birdies at the par-five 16th and the par-four 18th to end his day with a two-stroke advantage.
"Overall I felt like I was striking it quite nice," he said.
"I kind of got away with a few tee balls in the middle of the round where I wasn't hitting the fairways necessarily. ...But overall, I feel like my iron play has been pretty good lately ...Super pleased with the way I was hitting it."
Schauffele finished his round with a flourish, making birdies on his last two holes. The 10-time winner on the PGA Tour, who clinched the PGA Championship and the Open Championship in 2024, is seeking his first victory this year.
Amongst Australia's early starters, Min Woo Lee fired a two-under 70 that leaves him at two under. It proved a real mixed bag of a round for the 27-year-old, as his seven birdies were offset by five bogeys.
Veteran Adam Scott recorded a second consecutive level-par 72 but Karl Vilips had a calamitous 79, featuring two double-bogeys and one triple which ensured a weekend off.
World No. 2 and defending champion Rory McIlroy, looking to shrug off a back injury that led to his withdrawal from last week's Arnold Palmer Invitational, experienced another day of frustration with a one-under 71.
The Northern Irishman opened with a 74 on Thursday and is right on the projected cut-line.
Asked how his back was holding up, McIlroy replied: "It feels pretty much there. Not all the way there, but I feel like it's just progressively getting better each and every day."
with Reuters
