The former World Number One signed for a 1-under-par 70 at Quail Hollow in North Carolina courtesy of a run of 4-under in his last five holes to be in the mix just three off the pace.
At T15, Day is one shot and 10 places better than Adam Scott with another Queenslander, Scott Hend, next best at 1-over and T33.
Cameron Smith battled to a 4-over 75 as did Marc Leishman while Rod Pampling will have work to do to make the cut after a 6-over 77.
The final Australian in the field, Texas-based club professional Stuart Deane, signed for a 13-over 84.
But setting the pace among the Australians, Day’s late charge is an ominous sign for the rest of the field after the 2015 champion overcame a poor start to his round.
Hitting off the 10th, Day was in immediate trouble with a clumsy bogey at the opening par-5 and turned 1-over after swapping a birdie and another bogey at 14 and 16.
Things got worse after the turn with Day making back-to-back bogeys at the second and third to fall to 3-over for the day and well out of calculations.
But as he did so often during his extraordinary 2015 season, the 29-year-old flicked a switch with a birdie at the fifth followed in quick succession by an eagle-birdie run at the 7th and 8th to get under par for the day.
“Finishing with two birdies and an eagle is a really good turnaround,” he said after his round.
“From tee to green, it’s a challenging course but reasonably ... once you get on the greens, it’s a different beast.
“The greens are very quick and firm and if you’re not in the right position, it’s very hard just to make par.
“Especially if you find the greenside rough because the ball sits down.”
Day’s roller coaster round was the polar opposite of Scott who made one birdie and one bogey on each nine for his 71.
It was a familiar story for the 37-year-old who lost strokes on the greens to the rest of the field.
Hend’s round more closely resembled Day’s effort after he, too, started at the 10th though unlike his fellow Queenslander he opened with a birdie.
Three dropped shots to the turn, however, had him in trouble and he compounded the situation with a fourth bogey of the day at the second.
Birdies at the 5th and 7th salvaged his day, however, and he will head to Friday with some momentum in his first PGA Championship.
Smith and Leishman will both be less than pleased with their opening efforts, Leishman especially disappointed after decent finishes at the year’s first three majors.
The Arnold Palmer Invitational winner tackled the tougher back nine first and was behind the eight ball at the turn with a 4-over-40.
One birdie and one bogey on the inward journey saw him finish at that total and he and Smith will both need to improve to qualify for the weekend.