PALM HARBOR, 13 Mar 2015 (AFP) - The Englishman, a two-time winner on the European Tour, had Americans Sean O'Hair and Ricky Barnes hot on his heels.
Sweden's world number three Henrik Stenson, Alex Cejka of Germany, Zimbabwe's Brendon de Jonge and Americans Justin Thomas, Derek Ernst and Nicholas Thompson shared fourth on four-under 67.
Cejka is coming off a playoff victory on Sunday in the Puerto Rico Open.
Davis, seeking his first victory on the US tour, had stumbled late with bogeys at his 16th and 17th holes, the seventh and eighth at Innisbrook's Copperhead course.
He regained sole possession of the lead when he chipped in from 25 feet.
Davis had started strong with five birdies in his first nine holes.
He got to seven-under with birdies at the par-five fifth and par-four sixth, but three-putted for bogey at the seventh before a bogey at the par-three eighth, where he was unable to get up and down after missing the green.
He was just off the green at his closing hole, but got out of trouble with a perfectly judged chip.
"The golf course was there, no wind and fairly soft, so you have to try to make your score today if you could," Davis said.
O'Hair set an early target with a 66 that included eight birdies and three bogeys.
The 32-year-old, who won this title in 2008, is seeking his first victory since the 2011 Canadian Open.
He admitted that returning to a course where he has won before is a confidence builder.
"I've always liked this golf course," he said. "That's more where the confidence comes from... results from the past."
Barnes's six birdies included four in a row starting at the ninth, but his only bogey of the day, at the par-three 17th, cost him a share of the lead.
Stenson, playing at Innisbrook for the first time, even though he lives in Orlando -- about 100 miles (160 km) away -- was pleased with what he found, especially after a bogey-free round.
"Kept my patience and kept my game plan. All in all, a good day," he said.
Defending champion John Senden of Australia carded a three-over 74 that left him tied for a lowly 114th.
Leading first-round scores on Thursday in the US PGA Tour $5.9 million Valspar Championship (USA unless noted, par-71)
65 - Brian Davis (ENG)
66 - Sean O'Hair, Ricky Barnes
67 - Justin Thomas, Henrik Stenson (SWE), Derek Ernst, Brendon de Jonge (ZIM), Nicholas Thompson, Alex Cejka (GER)
68 - John Peterson, Kevin Streelman, Luke Guthrie, Shawn Stefani, Will Wilcox, Rod Pampling (AUS), Jason Kokrak, Ian Poulter (ENG), Andrew Svoboda, Adam Hadwin (CAN)
69 - Hudson Swafford, Billy Hurley, William McGirt, Ryan Moore, Ernie Els (RSA), Vijay Singh (FIJ), Lucas Glover, Jeff Overton, Russell Knox (SCO), Greg Chalmers (AUS), Jim Furyk, Harris English, Martin Laird (SCO), Stewart Cink, Kenny Perry, Will MacKenzie, Carlos Ortiz (MEX), Kyle Reifers