Marc Leishman of Australia hits his tee shot on the ninth hole during the second round of the BMW Championship at Conway Farms Golf Club on September 15, 2017 in Lake Forest, IllinoisMarc Leishman of Australia hits his tee shot on the ninth hole during the second round of the BMW Championship at Conway Farms Golf Club on September 15, 2017 in Lake Forest, Illinois (AFP Photo/ANDY LYONS)
After his only bogey of the day at the par-three 17th, Leishman birdied the par-five 18th at Conway Farms to complete 36 holes at 16-under par 126.
"I'm pretty happy with backing up a 62 with a 64," said Leishman, who started the day with a two stroke lead.
Four birdies on each side ensured Leishman finished the day atop the leaderboard despite the strong performances of Day and Fowler.
"I really took that as a challenge today, to not take it for granted that you're just going to make birdies," added Leishman, whose two US PGA Tour titles include this year's Arnold Palmer Invitational.
"You still have to earn every birdie. I think when you do get ahead of yourself, that's when bad stuff can happen."
Former world number one Day, seeking his first victory in more than a year, carded a six-under 65, benefiting from what he called a "lucky" hole-in-one at the par-three 17th -- where his tee shot hit in front of the green, bounced and rolled in.
"We were trying to land it 182 (yards) and I kind of pushed it," said Day, who won the 2015 title at Conway Farms. "It was a lucky hole-in-one, I guess.
"It's been, I think, 10 years since last time I had a hole-in-one," added Day, who had already chipped in for eagle at the par-five 14th. "I've had two eagles and a hole-in-one this week already, so everything's come in bunches."
Fowler also finished strong, with three birdies and an eagle from the 13th through 17th of a 64 for 129.
American Patrick Cantlay was alone in fourth after a 65 for 132.
British Open champion Jordan Spieth, who topped the FedEx Cup standings coming into the penultimate event of the US PGA Tour's four-tournament playoff series, carded a one-under par 70 to lie nine shots off the pace on 135.
Spieth played the first two rounds alongside PGA Championship winner Justin Thomas and world number one Dustin Johnson -- who came into the tournament second and third in the race for the $10 million playoff bonus that will be up for grabs at next week's Tour Championship.
Thomas also carded a 70 that left him in a group on 137 while Johnson closed with back-to-back bogeys in a 72 that left him languishing on one-over 143.
- McIlroy hopes fade -
It was a disappointing finish for defending champion Johnson, who needed two chips to get out of the greenside rough at 17, and was in the water at the final hole.
Northern Ireland's Rory McIlroy saw his slim hopes of defending the FedEx Cup playoff title dwindle further as his second-round 69 left him 15 off the lead.
McIlroy came into the tournament in 51st place in the playoff standings, with only the top 30 after this week's tournament advancing to the Tour Championship in Atlanta.
"It's hard because I know that realistically there's not much chance of getting in next week, but I still want to go out there and play well," McIlroy said. "I'm here, so I might as well try to play well and try to get the most out of myself."