James Hahn and Ryan Moore carded six-under 65s to lead the Deutsche Bank Championship after the opening round.
The American pair went around the TPC Boston on Friday bogey-free, moving into the lead by a shot at the second event of the FedEx Cup play-offs.
A group of four - Paul Casey, Fabian Gomez, Brian Stuard and Jhonattan Vegas - are a shot back at five under.
World number one Jason Day, also the leader of the play-offs, shot a one-under 70 before receiving bad news.
The Australian's wife and children were involved in a traffic accident, but everyone was "resting comfortably" afterwards according to Day.
On the course, bogeys at holes five and six proved costly for Day, who is tied for 48th.
Hahn started on the back nine and holed two birdies before four more followed, including to close out his round at the last.
"I feel like I'm rolling the rock pretty well, hitting a lot of fairways, that definitely helps," he said after his round.
"Staying patient [is important] and know that there's a lot of birdie opportunities out there and try and make a couple of extra putts."
Moore was also blemish-free, with three birdies on the front nine and another three on the back.
Jordan Spieth and Dustin Johnson are well-placed after the first round, shooting three-under 68s.
Defending champion Rickie Fowler is a shot behind that duo.