Herbert carded a sizzling nine-under-par 63 that featured six-straight birdies on the back nine to grab a two-stroke lead over South Korean S.H. Kim at the Fortinet Championship in Napa.

The Victorian carded a back-nine seven-under 30 with seven birdies in all on the Silverado Resort's North Course on Thursday to burst clear of the pack.

Americans Kelly Kraft, Jason Dufner, Zac Blair and Mark Hubbard, England's Harry Hall and Kim's countryman Sung Kang all share third spot at six-under, with another three players tied for ninth a shot further adrift.

Sydneysider Cam Davis is in a bunch at four-under, five shots behind Herbert, who believes he was merely shaking off the rust after taking time out following his missed cut at The Open in July.

"Golf's been getting me down pretty hard this year," he said.

"It was just a tough stretch there where I had a lot going on both in my life and on the golf course as well.

"Yeah, I missed the cut at The Open, I didn't really want to think about golf or talk about golf for about a good month there. Just needed to get away from the game and refresh everything.

"I've only been back on the Tour about a week now, so everything kind of feels like an adjustment out there at the moment. Still feel like I'm knocking the rust off a little bit." – Lucas Herbert.

"It sucked, I'd love to be here or up on the FedExCup standings as we speak, but hopefully taking that good break, refreshing, have a little reset gives me a better chance to play well in the big events next year."

Herbert was only one-under through a third of his round before catching fire on the greens to pick up eight shots in his last 12 holes."

"Found a little groove there," he said.

"It felt like anything I did poorly seemed to work out nicely for me and the good shots got rewarded as well.

"I've only been back on the Tour about a week now, so everything kind of feels like an adjustment out there at the moment. Still feel like I'm knocking the rust off a little bit."

At two-under, Harrison Endycott and Cameron Percy are the next-best-placed Australians behind Herbert and Davis.

Greg Chalmers signed for an even-par 72 while Aaron Baddeley will have some work to do to play the weekend in his 500th PGA Tour start, opening with a one-over 73 to be languishing in a tie for 86th.

Geoff Ogilvy had a day to forget, carding a four-over-par 76.