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, which was later equalled by American Brendon Todd, who is coached by former Aussie Tour player Brad Hughes.

Day and Tood lead by one stroke 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 former World No.1 Justin Rose.

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).

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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.

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

Scott recorded an impressive opening round of two under in his first competitive action since March. PHOTO: Harry How/Getty Images.

Day anticipated the cool temperatures in San Francisco and caked his back – which was operated on earlier this year – in Deep Heat 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've been working very hard in the off-weeks to get my putting back to where it is, because it's always been a strength of mine," Day said.

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

"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."

Scott admits he might not be as long as a bulked-up playing partner Bryson DeChambeau but says his own length off the tee is why he keeps contending in the US PGA Championship.

Round one was Scott's first competitive round since March and there were fears the 2013 Masters champion might be rusty for the first major of 2020.

The former World No.1 mixed three birdies with three bogeys on the front side before picking up two shots coming home.

"I'm very happy with my round. It wasn't really rusty, only a little bit," Scott said.

"If I can just correct that for tomorrow and get three or four more tee shots in the fairway, I can create some opportunities."

Scott witnessed a bizarre incident on Thursday when DeChambeau's unprecedented swing speed caused the shaft in his driver to snap shortly after his tee shot at the seventh hole.

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Scott is one of the PGA Tour's longer hitters but joked the media hype around DeChambeau's power was underwhelming in person.

"He's hitting it long; there's no doubt about it," Scott said after his round.

"Not to, like, put him down but I thought it was going to be longer (laughter). There's been so much build-up, me reading the news and listening to him and watching him play, I thought I was going to see like one of those long-drive guys.

"Fortunately, I still hit it long enough."

Recent form suggests a second major win could come at the PGA Championship for Scott, where he has finished third and eighth in the past two editions.

"I'm very happy with my round. It wasn't really rusty, only a little bit." - Adam Scott.

The US PGA Championship is renowned as the major that most favours big bombers, such as two-time defending champion Koepka.

But 40-year-old Scott knows he can still match it with the heavy hitters and combine it with his superb iron play.

"The (PGA Championship) courses have suited me the last couple years," Scott said. "They are difficult, but if you drive it well and you're solid with your irons and you can (contend)."

Marc Leishman and Matt Jones were the next best of the Aussie contingent after recording even par opening rounds of 70. Cameron Smith a shot further back at one over par, while Victorian Lucas Herbert currently sits below the projected cut line after opening with a three over 73.

- Evin Priest Australian Associated press, with additional reporting by Jimmy Emanuel