PGA Tour rookie Shawn Stefani had six birdies en route to a six-under 65 to seize a two-shot lead after the opening round of the $5.5 million Tampa Bay Championship on Thursday.
The 31-year-old American is coming off a season-best tie for 39th at the Puerto Rico Open last week.
Brian Harman posted a bogey on his final hole and is alone in second at four-under 67 on the Copperhead Course at Innisbrook Resort and Golf Club.
Harris English shot a three-under 68 and is tied in third with two other fellow Americans, Tag Ridings and Brendan Steele. English had been on course for second spot on the leaderboard before a double bogey on his last hole.
Two-time champion K.J. Choi, of South Korea, and Fiji's 2004 winner Vijay Singh both carded two-under 69s.
They are in a group of eight alongside Australian Geoff Ogilvy, Peter Tomasulo, Dicky Pride, Lucas Glover, Chez Reavie and Roberto Castro.
Defending champ Luke Donald, whose victory last year propelled him to top spot in the world rankings, stumbled home with three bogeys on the last four holes to finish at one-under 70.
"I am extremely disappointed," said Donald, who was four under through his first 14 holes. "I had it really under control and could have been six or seven under with a couple of putts.
"Obviously to finish like that, it leaves a bitter taste in my mouth."
England's Donald and Choi were joined in their group by 2010 winner Jim Furyk who shot a one-over 72.
Donald beat Furyk, South Korea's Bae Sang-moon and Robert Garrigus in a four-man playoff last year.
Donald is one of seven players ranked in the world top 20 in the field this week. He is also one of eight former champions participating, including South Africa's Retief Goosen, who won in 2003 and four years ago.
Goosen shot a disappointing five-over 76 Thursday.