Starting Friday two back of Leishman, 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.
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.
"[I am] very pleased with how the last two days went", Day said.
The BMW Championship brings back memories for Leishman, who was paired with golf icon and 14-time major victor Tiger Woods at the tournament in 2009 and it something he remembers fondly as he looks to add a third PGA Tour title to his name. "I think when you do get ahead of yourself, that's when bad stuff can happen". If the hole-in-one was lucky, the eagle was inch-perfect execution.
4 p.m.: Big par putt for Jason Day at No. 10 to stay T-2 at 15 under.
Day will donate the BMW M760i he won for his hole-in-one to a foundation that provides college scholarships to caddies, while BMW will also donate $US100,000 to the cause. You know, I've had two eagles and a hole-one-one this week already, so it's just...
It hasn't been easy for everyone, particularly defending champion Dustin Johnson.
Cantlay is playing his 11th tournament this year, yet he is No. 41 in the FedEx Cup and could get into the Tour Championship if he finishes in the top 30 after this week. He shot 69 and was at 7-under 135, in a tie for 12th. Phil Mickelson is trying to work his way into the top 30, and while he sputtered with two birdies, two bogeys and too many pars, he drilled an approach to 5 feet on the par-5 14th for an eagle.
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 penultimate event of the PGA Tour's $US67 million FedEx Cup series and will determine the all-important top-five seedings for next week's Tour Championship in Atlanta.
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 finished third two weeks ago at the TPC Boston and moved to No. 7 in the FedEx Cup.