Leishman was at 16-under 126, two short of the 36-hole record that Day set at Conway Farms two years ago on his way to a wire-to-wire, six-shot victory.
Day's tee shot at the 188-yard par-three 17th landed just in front of the green, took a couple of bounces and trickled into the hole, much to the Australian's surprise. Although in some ways, Jason Day has been on top at Conway Farms for the last 24 months.
Jason Day, playing for the first time using a high school friend as his caddie, was 7 under for his last 11 holes and shot 64. So, hopefully I can keep doing what I'm doing.
The second round of BMW Championship 2017 unfolds today (Friday, Sept. 15) in IL.
"I hit the ball really well but probably more importantly putted really well, made the opportunities I gave myself". BMW awarded $100,000 to the Evans Scholars Foundation, and then Day made a decision to give the vehicle he won to the Evans Scholars, which will yield another full, four-year scholarship for another student. He already has three eagles this week, two of them on Friday.
Fowler also finished strong, with three birdies and an eagle from the 13th through 17th of a 64 for 129.
American Patrick Cantlay is solo fourth at 10-under after a 65, while Italy's Francesco Molinari sits in fifth place at nine-under following his 65.
It hasn't been easy for defending champion Dustin Johnson, who is facing putting problems.
It's certainly been a lean year for Day, one that started with him as the world's top-ranked golfer and saw him show up to this week's event without a spot in the Tour Championship assured.
His 69 took him to 7-under 135, in a tie for 12th.
The world number 22 has won twice on the PGA Tour, including the Arnold Palmer Invitational in Florida in March.
The BMW Championship is the second last event in the lucrative FedEx Cup series.
The popular Northern Ireland golfer, now world No 6, will be looking for his first victory in the prestigious European Tour event, having finished runner-up in 2009, 2011 and 2014.
Leishman came into the BMW Championship ranked seventh in the FedEx Cup standings after finishing third at the Dell Technologies Championship.