Keegan Bradley carded three birdies in his last five holes to take a one-shot lead after the third round of the Travelers Championship.
The 2011 PGA Championship winner's nearest challengers are all fellow Americans, one of whom is Rickie Fowler who carded a career-low round but came inches short of joining the exclusive sub-60 club.
Bradley had shared the 36-hole lead with Denny McCarthy but by the end of the day Chez Reavie had emerged as his main rival after a second successive 63 lifted him to 20 under.
However, Bradley's blemish-free 64 ensured he stayed out in front on another day of low scoring - Patrick Cantlay moved into third place on 16 under with a 61 - where Fowler was the centre of attention.
Rory McIlroy finished nine shots off the lead after a frustrating round of 66, which included five birdies and a bogey at the par-five 15th, while Shane Lowry carded a second successive 69 to lie on eight-under in tied-33rd.
The 34-year-old Fowler, whose return to form saw him finish tied-fifth at last week's US Open at which he carded the joint-lowest round at a major (62), reeled off 10 birdies in 17 holes after starting at the 10th.
However, he could not hole his chip from the fringe at his final hole which would have seen him become the 13th player in PGA Tour history to break 60.
"I was very aware. I had a nice start to the day with a couple of good pars, made the turn at five (under) and at seven under through 11 I knew it was a legitimate shot," Fowler said in his post-round interview.
His birdie blitz did, however, catapult him to 15 under - the same score as McCarthy and Australia's Adam Scott - and gave him an outside chance of victory at TPC River Highlands in Connecticut.
