Day shot a hole-in-one on the par-three 17th on his way to a six-under 65, which has him tied with American Rickie Fowler for second spot.
Compatriot Day surged up the leaderboard with four birdies in a five-hole stretch around the turn, with his penultimate-hole ace one of two eagles over the back nine.
"It was a lucky hole-in-one, I guess".
"I know we have two days left so I can't really get too far ahead of myself".
To mark his ace BMW awarded Dollars 100,000 to the Evans Scholars Foundation and then Day chose to give the vehicle he won to the Evans Scholars, which will yield another full, four-year scholarship for another student.
"Confidence is high and hopefully I can just ride the wave", Leishman told reporters.
Fowler also chipped in for eagle on the reachable par-4 15th hole on his way to a 64. "I've had two eagles and a hole-in-one this week already, so everything's come in bunches".
"The ultimate goal is to win this week", Day said.
Lacking the magic he enjoyed over opening rounds of 62 and 64, the 33-year-old Leishman is fighting to maintain his cushion at the top with three birdies and two bogeys on moving day.
It hasn't been easy for defending champion Dustin Johnson, who is facing putting problems. Patrick Cantlay (65) is in fourth place but six shots behind. The world's No. 1 player can' seem to buy a putt, and even when he started to make a little progress, he finished bogey-bogey by taking two chips to get on the 17th green and hitting into the water on the 18th. The top points earner after next week's Tour Championship will receive a $10 million bonus awarded to the season-long FedExCup champion.
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 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.
McIlroy won the Tour Championship a year ago to scoop the lucrative FedEx Cup bonus, but world number two Jordan Spieth leads the rankings. Phil Mickelson and Charley Hoffman are the only players to reach the BMW Championship all 11 years of the FedExCup.