CHICAGO, 18 Sept 2015, (AFP) - The red-hot Aussie, who finished off a first-round 61 on Friday morning, signed for an eight-under par 63 for the second round at Conway Farms for an 18-under total of 124.
That put him five in front of US tour rookie Daniel Berger, who carded a 64, and Brendon Todd, who posted a 63 for 129.
Day also had a leg up on world number two Jordan Spieth and number one Rory McIlroy, who could see the currently third-ranked Australian leap-frog to the top of the world rankings with a victory on Sunday.
Spieth, who played alongside Day in the first two rounds, fired a second-round 66 for a share of fourth on 132, alongside fellow American Kevin Na, who also carded a 66.
McIlroy fired a 65 to head a group sharing ninth on 133.
Day, whose four victories this season include his first major title at the PGA Championship last month, had seven birdies and one bogey to go with his eagle in the second round.
His 124 matched the record 36-hole total set by Pat Perez in 2009 and equalled by David Toms in 2011.
But Day said he wouldn't be thinking about that as he headed into the weekend.
"It's over now," he said. "I've got to look ahead to tomorrow's round now."
Spieth, who also eagled the final hole to stay within striking distance, said Day had been so impressive in the two rounds they played together that it would have been worth the price of a ticket to watch.
"I feel like I should be paying to watch this event," he said.
Their day started when they returned to their final first-round hole, the par-four ninth, where Day needed to hole his second shot to shoot 59.
His 45-yard pitch shot from rain-soaked rough didn't make it and he settled for a par that still gave him a career-best round of 61.
"It sounds like everyone is disappointed," Day said. "I'm like, jeez."
While Day, 27, is chasing the world number one ranking, he already tops the standings in the US PGA Tour's FedEx Cup -- the playoff race that will be capped by next week's Tour Championship where a $10 million bonus will be on offer to the series winner.
He opened the playoffs by winning the Barclays on August 30.
Leading second-round scores on Friday in the $8.25 million BMW Championship (USA unless noted, par-71):
124 - Jason Day (AUS) 61-63
129 - Daniel Berger 65-64, Brendon Todd 66-63
131 - Kevin Na 65-66, Jordan Spieth 65-66
132 - Justin Thomas 65-67, George McNeill 67-65, Scott Piercy 67-65
133 - Harris English 65-68, Rory McIlroy (NIR) 68-65, Dustin Johnson 71-62
134 - Ryan Palmer 67-67, Henrik Stenson (SWE) 71-63, Brendon de Jonge (ZIM) 67-67, Matt Kuchar 67-67, Justin Rose (ENG) 70-64, Nick Watney 68-66, Keegan Bradley 68-66
135 - Ryan Moore 68-67, Hideki Matsuyama (JPN) 72-63, Bubba Watson 65-70, Sergio Garcia (ESP) 70-65, Brian Harman 66-69, Bill Haas 68-67, J.B. Holmes 70-65, Rickie Fowler 69-66, Brendan Steele 68-67, Kevin Chappell 66-69
136 - Tony Finau 72-64, Steven Bowditch (AUS) 70-66, Paul Casey (ENG) 67-69, Hunter Mahan 68-68, Cameron Tringale 72-64