Brendon Todd carded a career-low nine-under-par 61 to open up a two-shot lead after the third round of the PGA Tour Travelers Championship on Saturday.
Todd, chasing his third win of the 2019-2020 season, posted nine birdies in a bogey-free round at TPC River Highlands in Cromwell, Connecticut.
The 34-year-old had started the day four strokes off the pace after opening rounds of 66 and 65, but marched up the leaderboard with a flurry of five birdies over the front nine.
A red-hot putter ignited Todd's round from the first hole, delivering an opening birdie after making a tricky 13-footer.
Todd then made birdie putts of 25ft, 17ft and 10ft before rolling in a six-footer at the turn to move to five under for the day.
Four more birdies over the back nine left him at 18 under after three rounds, two clear of former world number one Dustin Johnson, who also swept up the leaderboard with a nine-under-par 61 of his own.
Todd had the chance to open up a three-shot lead heading into Sunday's final round, but saw his seven-foot birdie putt roll just past the cup on the 18th.
"I'm just riding the confidence I had last fall when I was playing well," said Todd, who won back-to-back tournaments last November.
Earlier, Johnson had made a bid for the lead with the lowest round of his PGA Tour career, finishing the day on 16 under after posting nine birdies and no bogeys.
The former world number one -- whose last PGA Tour win came at the WGC Mexico Championship in February last year -- was quickly into his stride with a birdie on the par-4 second after an approach left him six feet from the hole.
Another superb iron on the fourth left him with a 7-foot putt for birdie which he converted.
- Mickelson struggles -
He drained another 8ft putt for his third birdie of the day on the seventh and then improved to four under at the turn after his second shot on the ninth left him with a tap-in for birdie.
An 18ft birdie putt took him to five under on the 10th, and then back-to-back birdies on the 12th and 13th holes vaulted him into the lead on 14 under overall alongside Brendon Todd.
Johnson calmly rolled in a four-footer on the short par-four 15th for birdie and then took sole possession of the leaderboard with an 8ft putt for birdie on the 17th to put him at nine under.
Saturday's third round had teed off early in an effort to avert disruption from poor weather forecast for later on Saturday.
But the early tee off time did not agree with overnight leader Phil Mickelson, who is chasing a 45th PGA Tour title.
Mickelson, who had led by one after his seven-under-par 63 on Friday, was unable to reproduce that kind of momentum with a 71 that left him on 12 under, six off the lead.
The chasing pack behind Todd and Johnson on Sunday will be led by Kevin Streelman, who is three off the lead on 15 under after posting a seven-under-par 63.
Canada's Mackenzie Hughes is a further shot back on 14 under while Bryson DeChambeau and Kevin Na are lurking a shot back on 13 under.
Four other players are grouped with Mickelson on 12 under.
World number one Rory McIlroy endured a frustrating round however, closing with two bogeys over the final four holes to finish with a one-under-par 69.
That left McIlroy on 10 under for the tournament, eight shots adrift of leader Todd.
Elsewhere on Saturday, the spectre of coronavirus continued to hover over the tournament as Jason Day played a solo round.
Day had teed off alone after requesting a COVID-19 test as a precautionary measure. The Aussie was later confirmed to be negative for the disease, which had already forced the withdrawal of several players from the tournament.