Jonathan Byrd seized a one-stroke lead over fellow American Pat Perez after a blistering back nine in the third round of the Wells Fargo Championship.
On an emotional day after the death of Spanish legend Seve Ballesteros, Byrd birdied five of six holes after making the turn on his way to a five-under 67.
Overnight leader Perez had held a two-shot lead over Byrd before a bogey at the par-four 12th.
Byrd on 15 under is five shots clear of England's Brian Davis, who shot 69.
Ireland's Padraig Harrington and Englishman Justin Rose also carded scores of 69 in the third round to leave them on six and three under respectively.
Byrd had back-to-back birdies from the seventh before making bogey at the ninth.
And with his surge after the turn, he had seven birdies during a nine-hole stretch.
"Wow, I did that? I didn't really realise it was going that good there for a while," said the 33-year-old, who won the Tournament of Champions in January.
He added that it took a little while to get accustomed to playing with Phil Mickelson, who shot 74.
"I was pretty shaky starting off and didn't feel real comfortable out there," Byrd admitted.
Beyond the birdies and bogeys, many players were shaken by the death of five-time major winner Ballesteros.
The Spaniard died early on Saturday morning at the age of 54 after a battle with brain cancer.
Fiji's Vijay Singh, who shot 73, reflected: "He was a great man, not only on the golf course but off the golf course.
"I think not only people that knew him through golf, but everybody that knew him through what he did for golf, I think it's a great loss and he'll be missed."
Ballesteros mentored compatriot Sergio Garcia, who struggled at the end of the round with two double bogeys and a bogey over his last five holes for 74.
"It's a sad day," Garcia commented. "We're all feeling for Seve's family. He's been suffering for quite a while now, so it's probably better that he's resting now."
Garcia said he was fortunate to have known Ballesteros and added: "He was special. He was just such an amazing player, such an amazing person."
Stewart Cink, who won the Open in 2009, is three shots off the lead after carding a 68. Cink is level with fellow American Lucas Glover after the 2009 US Open winner shot 69.
Andres Romero of Argentina (67) and Americans Kevin Na (67) and Bill Haas (71) are four behind the leader.