Leishman, who goes into the weekend with a three-shot lead over Day and Fowler, told the PGA website: 'The game's feeling really good.
Day made a hole-in-one at the par-three 17th and Fowler carded a pair of eagles to stay in contention on 13 under.
Lahiri was done in by a disappointing finish over last three holes, where he bogeyed three times including two on the last holes in the second round.
Leishman, seventh in the FedEx Cup rankings before the tournament, dropped only one shot for a second successive day.
"I really took that as a challenge today, to not take it for granted that you're just going to make birdies", Leishman said.
The second round of BMW Championship 2017 unfolds today (Friday, Sept. 15) in IL.
"Just like yesterday I gave myself heaps of opportunities and took advantage of them so happy to back up a 62 with a 64". Jamie Lovemark and Charley Hoffman also opened at 64.
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.
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.
He already has 18 birdies in 36 holes at Conway Farms, where the scoring average was a shade under 69 through two rounds.
The Northern Irishman needs a top-four finish to reach the Tour Championship, the final Tour event of the season. 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.
Johnson won the first playoff event; Thomas, the second.
It's easy to overlook Leishman because the Australian has only two PGA Tour victories, including the Arnold Palmer Invitational earlier this year.
Leishman is now a projected 4th in the Fedex Cup points table and if he can remain inside the top four heading into next week's Tour Championship then he controls his own destiny in search of the US$10 million FedEx Cup bonus as any player inside the top five would win the top prize with victory in Atlanta.
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