Australia's Jason Day moved joint top of the leaderboard alongside overnight leader Ryan Palmer following the second round at the Deutsche Bank Championship.
The 26-year-old, in search of his third PGA Tour triumph, carded a 68 to add to his opening 66 in Boston to move into a one-shot lead with Palmer on eight under par.
Day recorded a flawless front nine where he claimed five birdies, but his back nine let him down and he could have taken the outright lead at the halfway stage had he not suffered with bogeys on the 12th, 14th and 18th holes to go three-under for the round.
American Palmer also endured a mixed round and a double bogey on the second could have meant trouble, but he claimed five birdies among three bogeys to finish level on par and in the lead for the second day.
One shot further back are Palmer's compatriots Matt Kuchar and Billy Horschel, who both enjoyed an impressive round of 66 to stay in contention.
Kuchar, who hit a 69 on Friday, admitted he was struggling on day one after hearing of the sudden death of his caddie Lance Bennett's wife Angela two days earlier after suffering a seizure.
He told www.pgatour.com: "Today was easier. I think that's the natural progression of how these things work. (I) still felt like Angela was on my mind almost every hole, every shot. But yesterday I had a hard time following through a couple of times."
Horschel's round was almost flawless, a bogey on the eighth the only blemish among six birdies.
Five Americans shard fifth place, Ryder Cup qualifier Patrick Reed joined by Keegan Bradley, Bill Haas, Webb Simpson and Russell Henley.
Three shots off the lead sits Scotland's Russell Knox in a tie for 10th, one shot behind 2011 winner Webb Simpson following a second-round 70 which included a three birdies and a double bogey on the 10th which set him back.
World number one Rory McIlroy was two under ahead of the third round, the PGA Championship and Open Championship winner improving his first-round score by one shot with 69 on a scorecard including three birdies and a bogey on the front nine, but a double bogey on the 14th set him back before he ended the round with two further birdies.
The Northern Irishman sits one shot ahead of England's Ian Poulter, who struggled after his opening round 67 as he carded a 73 which included a triple bogey on the 16th hole and a double bogey on the 1st - among five birdies and two bogeys - to drop back in the field.
The shot of the day came from a player who ended up missing the cut, Robert Garrigus sending his 50-foot putt off the fifth green and onto the fringe to allow the ball to catch a slope to the left and roll in.
He could only manage a second successive 73 and finished one shot outside the cut-off on four over.