Jason Day has fired the lowest opening round of his US PGA Championship career to claim the clubhouse lead at golf's first major of 2020, while fellow Australian Adam Scott is close behind.

Lathered in Deep Heat to protect his ailing back, Day birdied his last hole for a brilliant bogey-free five-under-par 65 at TPC Harding Park to snare a one-shot lead.

Day led by one stroke early from a large group at four-under headlined by defending champion Brooks Koepka, Germany's former winner Martin Kaymer, two-time major winner Zach Johnson and fellow former world number one Justin Rose.

Brendon Todd ended up tying for a share of the lead late on the first day.

Tiger Woods began his search for a 16th career major in style with a solid 68 to sit three behind Day at two-under, and was later joined by Queenslander Scott (68).

Playing his first tournament since March, a rusty Scott mixed three birdies with three bogeys on the front side before picking up two shots coming home to climb into contention.

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Dual winner Rory McIlroy sits at even-par after a 70.

Day anticipated the cool temperatures in San Francisco and caked his back — which was operated on earlier this year — with muscle therapy cream to prevent another injury.

"I pretty much lather up in Deep Heat and I try and burn the skin off my back, to be honest," Day said after his round. "I feel pretty good."

Day teed off from the 10th hole and began with a birdie at the par-5. He added another at the par-3 17th to make the turn at two-under.

He then rattled off birdies at the first, fourth and ninth holes to card a blemish-free start in search of his second career major.

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"I drove it really nicely and, when I was out of position, I left myself on the right side of the fairways," Day said.

Day's five birdies came from close range with his longest birdie putt coming from 13 feet.

The 12-time PGA Tour winner hasn't had to tap in to his usually world-beating form on the greens, but feels it's ready when he needs it.

"I feel like the game is slowly coming around and the confidence is coming around because I'm starting to see the results, which has been good."

It was just the second tournament since February for four-time PGA champion Woods, but the 44-year-old dismissed any concerns of rust for the majors season.

"I felt that practice sessions [at home] have been very good," Woods said. "They have been very intense, so there was no reason why I can't [hit the ground running]. [Today] was a very solid round."

At one-under, Cameron Smith was next best of the Australians nearing completion of his round.

Smith's countrymen Matt Jones, Lucas Herbert and Marc Leishman were all even with the card.

Beefed-up Bryson DeChambeau breaks driver

In the afternoon session, Bryson DeChambeau — whose bulked-up physique has been the talk of the tour since the return from the COVID-19 break — broke his driver, which he has nicknamed "the Kraken," after teeing off on the seventh hole.

DeChambeau hit what appeared to be a normal tee shot 296 yards into the rough on the par-4 hole. But when he bent over to pick up his tee, the club snapped under him.

A rules official said he would be allowed to replace the club during the round, and a member of DeChambeau's support team fetched another shaft for him.

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It was because the club had not been damaged through anger or abuse that DeChambeau was permitted to get a new one.

"During the course of playing the stroke the club busted into pieces and we do have the local rule in effect when that happens, the player would be allowed to replace the club," rules official Brad Gregory explained to PGATour.com.

"Starting in 2019 this is a local rule that's in effect for the Championship for when a club is significantly damaged. And breaking into pieces would meet that standard."

"That was weird," DeChambeau said as he walked toward the green, still holding the pieces. "I guess it's all those swings I put in."

DeChambeau finished on two-under for the day after shooting a 68.

ABC/wires