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.
He did that and then some, his confidence boosted by his birdie at the par-four seventh, where he struck a low fizzing pitch that scuttled into the hole.
"We were trying to land it 182 (yards) and I kind of pushed it", said Day, who won the 2015 title at Conway Farms. The Clemson alum landed seven (of 14) fairways and 13 greens in regulation, losing 1.280 strokes off-the-tee but gaining 2.777 around-the-green. "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". Last time I was here I was five shots better [after 36 holes].
"Last week showed that leading by a couple of shots going into the back nine is not enough and there is a lot of great players just behind me so I need to keep the foot on the pedal. That's the main goal".
The BMW Championship is the third of four playoff events on the PGA Tour.
Leishman has been the form Australian on Tour this year with a win and six top-10 results, but the 33-year-old Victorian says he'll draw inspiration from a memorable round in Chicago with Tiger Woods as he chases a third PGA Tour victory.
Jordan Spieth, No. 1 in the FedEx Cup after successive runner-up finishes in the playoff events, only managed a 70 and joined Mickelson in the group at 135.
Marc Leishman left his clubs in the garage during his week break from the FedEx Cup playoffs and it didn't make a difference.
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. "It isn't quite coming to together, it's about being patient".
McIlroy had another up and down day, sinking five birdies, but giving shots back with a double bogey on six and a bogey on 13. He looked away briefly before the ball hit the fringe and kicked forward toward the hole before rolling in.