Boston - Ryan Moore and James Hahn fired six-under-par 65s to grab a one-shot lead after a low-scoring first round at the Deutsche Bank Championship at TPC Boston on Friday.
Hahn drained an eight-foot birdie putt on his final hole while Moore saved par on his last to inch clear of the chasing pack in the $8.5 million tournament, the second event in the FedExCup playoff series.
Brian Stuard, England's Paul Casey, Argentina's Fabian Gomez and Venezuela's Jhonattan Vegas were locked in a four-way tie for third at five under after carding 66s.
Australia's Adam Scott - winner of the inaugural Deutsche Bank crown in 2003 - was amongst a bunch of 13 players tied for seventh, two strokes off the pace.
Moore and Hahn opened up their slender advantage after recording bogey-free rounds with six birdies apiece on the demanding 7,207-yard layout.
World number 43 Moore is chasing his sixth PGA Tour title, just a month after winning the John Deere Classic by two shots.
The 33-year-old is 11th in the lucrative FedExCup standings could climb to second overall with a victory this weekend.
Moore admitted he was at a loss to explain the unusually congested field at the top of the leaderboard, where only two shots separate 19 players.
"It's hard to say why that happens," said Moore. "Usually someone will kind of separate and maybe shoot an eight under or nine under sometimes around this course."
Hahn meanwhile enjoyed a near flawless round, hitting every fairway in regulation for the first time in his professional career.
The 34-year-old Korean-born world number 65 is seeking the third win of his PGA Tour career, with the most recent victory coming at the Wells Fargo Championship in May.
Casey and Gomez had set the early pace in the morning flights, although Stuard briefly went to six under before faltering with back-to-back bogeys before finishing with a birdie.
World number two Dustin Johnson, number three Jordan Spieth, Sweden's number four Henrik Stenson, Patrick Reed and Olympic champion Justin Rose of England are all three shots adrift.
Reed is well-positioned for a charge over the weekend as he looks to build on last week's victory at The Barclays.
Defending Deutsche Bank champion and world number eight Rickie Fowler is four off the lead, tied for 38th, after carding four birdies and a pair of bogeys.
World number one Jason Day of Australia is also in touch at one under after an eventful day which saw his wife Ellie and children escape injury in a traffic accident.
Ellie Day was taken to hospital as a precaution but later released.
"Thankfully everyone is now resting comfortably," said Day, who is tied for 48th with Fiji's Vijay Singh and 2014 Deutsche Bank winner Chris Kirk among others.
Northern Ireland's world number five Rory McIlroy and Bubba Watson are tied for 67th six shots back.
But it was a miserable day for Phil Mickelson, who ended up carding a quadruple bogey at the 439-yard par four sixth hole. Mickelson finally finished 10 strokes off the lead, tied for 93rd.
Leading scores after first round of Deutsche Bank Championship at TPC Boston on Friday (par-71, USA unless stated):
65 - James Hahn, Ryan Moore
66 - Paul Casey (ENG), Fabian Gomez (ARG), Brian Stuard, Jhonattan Vegas (VEN)
- Kevin Chappell, Jim Furyk, Steve Stricker, Roberto Castro, Harris
English, J.B. Holmes, Tony Finau, Daniel Berger, Adam Scott (AUS),
Emiliano Grillo (ARG), Kim Si-Woo (KOR), Billy Hurley III, Chad Campbell
- Jerry Kelly, Marc Leishman (AUS), Justin Rose (ENG), Jimmy Walker,
Henrik Stenson (SWE), Patrick Reed, Dustin Johnson, Brian Harman, Zach
Johnson, Kevin Streelman, Jamie Lovemark, Jim Herman, Smylie Kaufman,
Sergio Garcia (ESP), Jordan Spieth, Vaughn Taylor, David Hearn (CAN),
Adam Hadwin (CAN)