PALM HARBOR, Florida, March 15, 2013 (AFP) - Choi opened with an eagle on his way to firing a four-under par 67 while Scott fired a bogey-free 66, but Stefani's 70 was enough to keep him ahead on seven-under par 135 after the second round at the $5.5 million event.
Scott and Choi shared second on 136 with Americans Brian Harman, Harris English, Jason Dufner and Peter Tomasulo another stroke off the pace and Spain's Sergio Garcia and American Tag Ridings sharing eighth on 138.
Choi, who won the Tampa Bay title in 2002 and 2006, followed his stellar start with a birdie at the sixth but stumbled with a bogey at the par-3 eighth.
Birdies at the par-5 11th and par-3 13th and 15th holes lifted Choi into a share of the lead, but a bogey at the par-4 16th dropped him back as he bids for his first US tour triumph since the 2011 Players Championship.
"This course is very comfortable for me," Choi said.
Scott, coming off a share of third last week at Doral, birdied all four par-5 holes on the Innisbrook Copperhead layout -- the first, fifth, 11th and 14th -- and birdied the par-3 fourth as well.
"To go bogey-free anywhere is good," Scott said. "Here is particularly good. It is quite a tricky golf course and mistakes are easily made."
Scott, who made bogeys on the last four holes to fall to Ernie Els at last year's British Open, last won a US event at the World Golf Championships event at Firestone Country Club in 2011.
Stefani, who has not cracked the top 35 at a PGA event, birdied the sixth hole, took a bogey at seven, then answered with a birdie at the eighth. After starting the back nine with a bogey, he birdied the 15th and parred in.
"The lead is one thing but leading on the weekend is another," Stefani said. "I just wanted to go out there and play golf and hit each shot the best I could and really have some fun. That's really what I did. I really had a lot of fun out there, even though I didn't really play my best."
South Africa's George Coetzee, Australian Greg Chalmers and American Ben Kohles shared 10th on 139 with England's Brian Davis and Aussie Mark Leishman among nine others on 140.
Leading scores on Friday after the second round of the $5.5 million US PGA Tampa Bay Championship (USA unless noted, par 71):
135 - Shawn Stefani 65-70
136 - Adam Scott (AUS) 70-66, KJ Choi (KOR) 69-67
137 - Brian Harman 67-70, Harris English 68-69, Jason Dufner 71-66, Peter Tomasulo 69-68
138 - Tag Ridings 68-70, Sergio Garcia (ESP) 71-67
139 - George Coetzee (RSA) 71-68, Ben Kohles 72-67, Greg Chalmers (AUS) 71-68
140 - Mark Leishman (AUS) 70-70, Justin Leonard 71-69, Scott Brown 70-70, Matt Kuchar 72-68, Jerry Kelly 70-70, James Driscoll 74-66, Jordan Spieth 72-68, Erik Compton 75-65, Brian Davis (ENG) 71-69
141 - Cameron Tringale 71-70, Jimmy Walker 73-68, Geoff Ogilvy (AUS) 69-72, Sean O'Hair 73-68, Justin Hicks 70-71, Charley Hoffman 71-70, Ryan Palmer 71-70, Jim Furyk 72-69, Bryce Molder 72-69