A visibly relaxed Day fired a six-under-par 65 at Monterey Peninsula Country Club to sit just one shot off a two-way tie for the first-round lead held by Americans Scott Langley and Brian Gay, who each shot 64 also at Monterey.

Teeing off the 10th hole, Day carded two early birdies before a bogey at the par-4 15th.

However the 13-time PGA Tour winner responded with three straight birdies before the turn and added two more on his back nine.

With Day and wife Ellie welcoming their third child, son Arrow, in November and Day enjoying a run without injuries that have recently plagued him, the Queenslander says he has never felt clearer.

He says it's playing a large part in his solid start to 2019, which also kicked off with a top-five finish at Torrey Pines last month.

Now, the 31-year-old has given himself a hot start to the Pebble Beach Pro-Am – where he has recorded five top-six results, including a runner-up last year, without lifting the trophy.

"I've just got a clearer head," the former World No.1 said.

"When you don't have any distractions and you have a clearer mind, you can focus on playing. Things start to align and become more balanced and that definitely yields good (golf).

"I'm very close to getting back to where I need to be."

Day plays from one of the bunkers of the 4th hole during the opening round of the 2019 Pebble Beach Pro-Am. PHOTO: Jeff Gross/Getty Images.

Day shares third at six under alongside five others including five-time major winner Phil Mickelson.

Mickelson is regarded as having arguably the greatest short game in golfing history but has perennially struggled with the driver.

However the 48-year-old left-hander hit all 13 fairways at Monterey – a milestone he says is a career-first.

"So history was made today; to the best of my knowledge it's taken me 27 years and a few months to hit all fairways in a single round in a competition," Mickelson said.

Meanwhile, Aaron Baddeley was next best of the Australians with a three-under-par 69 at the difficult Spyglass Hill leaving him just four shots off the pace.

Baddeley's countrymen Adam Scott, Curtis Luck and Matt Jones sit one under after day one, while John Senden, Rod Pampling, Cameron Davis and Ryan Ruffels had even-par rounds.

Severe rain is forecast for California's Monterey Peninsula during the next three days and PGA Tour officials are bracing for a Monday finish.

- Evin Priest, Australian Associated Press