Smith is sitting tied sixth, four shots behind Chile's Joaquin Niemann, who has a three-shot lead after a brilliant 63 at beautiful Riviera in Los Angeles on Thursday.

But the Australian No.1 still wasn't content with his strong start in a field containing all of the world's top-10 players.

"It was a good grind today, absolutely. I got out a really good score. Probably didn't deserve four-under, but some days golf does that to you," Smith said.

Joaquin Niemann leads by three at the Genesis Invitational after a brilliant opening round. PHOTO: Michael Owens/Getty Images.

"I scrambled really well. I hit my irons really good as well. It just wasn't my day with the driver.

"I'll just go down to the range and work something out for tomorrow. The rest of the game feels really good."

Good enough to mix five birdies with a lone bogey on Thursday.

Niemann, though, bagged nine birdies in easily the round of the day.

"I think it's got to be one of my best days on the golf course, especially in a place like this with this history," Niemann said.

"Riviera Country Club is one of my favourite golf courses and this event is my favourite during the year."

"It was a good grind today, absolutely. I got out a really good score. Probably didn't deserve four-under, but some days golf does that to you." - Cam Smith.

He leads from last week's Phoenix Open winner Scottie Scheffler and fellow Americans Jordan Spieth, Max Homa and Cameron Young, who all carded 66s.

Three-time major champion Spieth also paid tribute to the historic course.

"I love Riviera. I think it's arguably, it's in the conversation as the best golf course in the world," Spieth said.

Smith shares sixth with CT Pan, Jason Kokrak and Open champion Collin Morikawa, who can seize Jon Rahm's World No.1 ranking with a victory on Sunday.

Two-time Riviera champion Adam Scott finished with a flurry of three straight birdies to be the next-best Australian at three-under 68, one behind Smith in a share of 12th spot.

Marc Leishman and Matt Jones both carded one-under 70s to be tied for 43rd.

Rahm shot a 69, as did Rory McIlroy, while Dustin Johnson, who has slipped to sixth in the world, turned in a 73.