Starting the day two back of Leishman, who opened with a nine-under 62, Day bounced back from a sloppy start featuring two bogeys with three birdies in his last four holes on the front side, before picking up an additional three shots on the inward half.
It carried day to a 65, putting him in the last group on the weekend with Leishman.
"My last hole-in-one was at Turning Stone (Resort Championship) in 2008, so I've had a bit of a dry spell", Day said after his round. He made a hole-in-one on the 17th hole with a 7-iron that turned into a payoff for multiple parties.
"I hit the ball really well but probably more importantly putted really well, made the opportunities I gave myself".
The generous Day then donated the BMW M760i he won for the ace to the Evans Scholar foundation which provides college scholarships to caddies.
"It's very quick down there so I just wanted to get something just landing on the green", he said.
Sing it people... 'To the left to the left... to the right to the right...' Jason goes commando at BMW
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. You know, I've had two eagles and a hole-one-one this week already, so it's just... Defending champion: Dustin Johnson.
Fowler also finished strong, with three birdies and an eagle from the 13th through 17th of a 64 for 129.
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.
It's easy to overlook Leishman because the Australian has only two PGA Tour victories, including the Arnold Palmer Invitational earlier this year.
Leishman came into the BMW Championship ranked seventh in the FedEx Cup standings after finishing third at the Dell Technologies Championship.
Leishman is now a projected 4th in the Fedex Cup points table and if he can remain inside the top four heading into next week's Tour Championship then he controls his own destiny in search of the US$10 million FedEx Cup bonus as any player inside the top five would win the top prize with victory in Atlanta.