Adam Scott has charged into contention at the US PGA’s Tampa Bay Championship, but American rookie Shawn Stefani is clinging to a one-shot halfway lead over the Australian No.1 and South Korea’s KJ Choi.

Scott fired a bogey-free round of five-under 66 and Choi opened with an eagle on his way to a 67, but Stefani’s 70 was enough to keep him ahead at seven-under at the $US5.5 million ($A5.32 million) event.

Scott and Choi share second at six-under 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 a further stroke adrift.

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-three 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 last won a US event at the World Golf Championships event at Firestone Country Club in 2011.

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-three eighth.

Birdies at the par-five 11th and par-three 13th and 15th holes lifted Choi into a share of the lead, but a bogey at the par-four 16th dropped him back as he bids for his first US tour triumph since the 2011 Players Championship.

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.”