Rose, 38, grabbed six birdies over his opening nine holes ahead of posting nine straight inward half pars in a round of 64 for a 17-under par leading tally on the Aronimink course.
The Bahamas-based Brit is no stranger to winning in Philadelphia having captured the 2010 AT&T National, and the second of now nine Tour victories, on this week’s host.
Then in 2013 the current World No4-ranked Rose captured a first major in winning the U.S. Open at Merion, located just seven miles from Aronimink.
“It was a good day for me today and while it was a day I felt I didn’t play that well, it was a good day for scoring and I made my score on the front nine,” he said.
“I know there will be guys behind me on the leaderboard who will be gunning to play well and there will be guys who will just play well.
“So, a one-shot lead doesn’t really mean much and it doesn’t change my approach tomorrow as I just have to play better than the guys around me.”
American Xander Schauffele, who led the $9m event after a second-round 64, was tied with Rose until the Florida golfer bogeyed the last in a score of 67.
Schauffele, 24, fell back into a share of second with four-time Major winner Rory McIlroy, who brilliantly regrouped after a ‘no frills’ second day 69, to grab a ninth-hole eagle and seven birdies in his score of 63.
Justin Rose soars into the lead at the BMW Championships
Xander Schauffele is tied second on the leaderboard with Rory McIlroy
If you didn’t shoot 65 you were losing ground
McIlroy’s only hiccup was again making a mess – for a third day running – of the par-3 eighth hole, posting a double-bogey after two earlier bogeys.
“I had missed a couple of shots on Friday but today I made the putts I needed to while I also hit more fairways that gave myself more opportunities,” said McIlroy.
“But then it was a weird day as they put a lot of the tees forward and while the course was very wet from all the rain we had last night, if you didn’t shoot 65 you were losing ground.
“So, it was just one of those days you had to stay aggressive.”
England’s Tommy Fleetwood made it three European Ryder Cup stars in the leading four, shooting a second straight 62 to be tied at 15-under with rival Rickie Fowler (65).
Fleetwood grabbed eight birdies over each of the two rounds including birdying five of the same holes he had done on Friday.
“If I had to choose which was the better 62 I would take Friday’s as I just seemed to hit the ball marginally better, but then who’s going to complain when you shoot a 62,” said Fleetwood.
Fowler remains a picture of consistency in his first event back since injury, signing for a third straight 65.
Open champion Francesco Molinari (64) made it five Europeans in the top seven to be at 13-under and a shot fewer than Tiger Woods (66)
Woods produced his best start in the three days with back-to-back birdies and moved into double figures at 10-under par but the birdies dried up on the rain-softened course, with Woods producing 11 straight pars before a birdie at the 14th.
Woods then holed a five-footer at the par-5 16th to be four-under on the hole over the three days of the $US 9m event and then end with a pair of pars.
“Today was one of those days where it was really… It was frustrating because I hit the ball well enough to shoot a low score,” said Woods.
“I got off to a quick start, birdying the first two, and Joey (Le Cava – caddy) was tripping in my ear to stay patient out there because I was getting a little hot.
“I was looking up on the board and everyone seemed like they were two-under through eight holes, five-under through nine but I was only two-under, and I wasn’t doing much.
“I’m now going to have to shoot one of those 61s or 62s just to have a chance. The golf course is soft and everyone is making birdies. You can be aggressive. I got to take a run at it, whether it’s tomorrow or Monday.”
Rory McIlroy is tied second and praised the class of the field
One shot further back is Tony Finau (67), trailing six behind Schauffele and his USA Ryder Cup ‘wildcard’ pick rival.
Tour officials also have moved the final tee-times forward to a 7am start (local time) for a second occasion this week due to the threat of Sunday afternoon thunderstorms in Philadelphia.
Players will tee-of in groups of three and from both the first and 10th tees.
Should play not be completed the Tour indicated they will go to a Monday finish.