Day's 13-under total leaves him with a share of the clubhouse lead alongside world No.10 Rickie Fowler (64) and countryman Leishman, who has picked up four shots through 12 holes.
"My last hole-in-one was at Turning Stone 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. Day also won a auto and, not long after signing his card, he chose to donate that to the foundation to pay for another full scholarship.
McIlroy said: "I've finished second three times, so perhaps this is the year I can go one better and end my season on a real high".
Marc Leishman opened up a three-stroke lead after two rounds of the BMW Championship as Rory McIlroy's FedEx Cup hopes faded. "Seeing the last two days, I know I still have it in myself to be able to hit the correct shots and go out there and try and win".
Fowler also chipped in for eagle on the reachable par-4 15th hole on his way to a 64.
The 29-year-old came back from three bogeys to put in another spectacular performance with an eagle on the 14th, ending round two with a six-under 65.
"The ultimate goal is to win this week", Day said.
It hasn't been easy for everyone, particularly defending champion Dustin Johnson.
America's Patrick Cantlay is in fourth position at 10-under par after a 65, while Francesco Molinari (65) of Italy lies a shot further back at nine-under. The top points earner after next week's Tour Championship will receive a $10 million bonus awarded to the season-long FedExCup champion.
Jordan Spieth had a bogey-free 65, while Phil Mickelson shot 66. He shot 69 and was at 7-under 135, in a tie for 12th.
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.
The powerful finish allowed Day to make a massive late move through the field and he will start round three just two off Leishman's pace. It's an ebb and flow for players at the top level, all of whom vacillate throughout the year while trying to find the sparse middle ground that yields both ample focus and a sufficiently loose approach.