Day's round was highlighted by his hole in one at the par three 13th which followed yet another eagle at the par five 14th which came after he holed form just off the green. Day was two-under par heading onto hole No. 14.
Fellow Australian and final group playing partner Jason Day, the 2015 BMW victor, is one-under for the day in outright third at 14-under.
To hear him talk of his wire-to-wire victory at the 2015 BMW Championship that took him to No. 1 in the world is to listen to someone describing an out-of-body experience. So, hopefully I can keep doing what I'm doing.
"I hit the ball really well but probably more importantly putted really well, made the opportunities I gave myself".
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.
To mark his ace BMW awarded Dollars 100,000 to the Evans Scholars Foundation and then Day chose to give the auto he won to the Evans Scholars, which will yield another full, four-year scholarship for another student. Jamie Lovemark and Charley Hoffman also opened at 64. A bogey on 13th was made up with a birdie on 15th, but closing bogeys on 17th and 18th spoilt his card. 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 defending champion Dustin Johnson, who is facing putting problems.
His 69 took him to 7-under 135, in a tie for 12th.
The 28-year-old Northern Irishman captured the FedEx Cup title last year - and its $10 million bonus jackpot - but is now projected to finish 60th unless he makes a big move over the final two round, estimating he needed "a couple of 65s" to make any impact.
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.