Brooks Koepka and Bryson DeChambeau each shot a six-under 66 to take a tie for the first-round lead at the Masters at Augusta National.

Aussie Adam Scott was for a while tied for the lead after completing his first round at the US Masters on a big day for Aussies while Tiger Woods also produced a brilliant back nine to remind everyone of his threat as he aims to capture another major.

Scott's place atop the leaderboard didn't last, eventually slipping back to be tied for sixth at three-under, as Ian Poulter and Koepka made some big plays. Poulter ended on four-under after birdies on 15 and 16 but was soon overtaken by Koepka, who caught fire and sunk five birdies in six holes on the back nine to assume the lead at six-under.

Three consecutive birdies to DeChambeau saw the American make a stunning run to sit level with Koepka atop the leaderboard before a cruel, close call on the 18th prevented him from taking the outright lead.

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His incredible approach shot hit the flag and bounced back out to finish inches from the hole before he tapped in for a fourth straight birdie to end at six-under and cap a remarkable day. It was agony and ecstasy all at once for the World No. 6.

The near-miss came after DeChambeau almost aced the 16th but the 25-year-old will still be thrilled with his astonishing opening round 66 — his lowest ever round at Augusta.

DeChambeau couldn't see what happened on 18 and only saw his ball hit the flag stick when he was shown a replay during a post-round TV interview, and he threw his head back and laughed when the full story became evident.

"Oh my gosh, I guess I should have pulled the flag stick," DeChambeau said.

Dustin Johnson jumped to four-under after birdies on the 13th and 16th while Phil Mickelson went on a late rampage to birdie five of his last seven holes to shoot 67 — his lowest first round score at Augusta since 2010 — as he sat third on five-under.

South African Justin Harding and Spain's Jon Rahm joined Scott on three-under after the opening 18 holes of the prestigious tournament while Woods finished the day one shot back at two-under par, as did Rickie Fowler.

At 33, Harding is the oldest of 17 rookies in the field and surpassed his own expectations for his first-up performance. "It was good fun, I thoroughly enjoyed it," Harding said. "I handled the emotions of the day very well — better than I expected."

Kevin Kisner and Kiradech Aphibarnrat also finished at three-under.

Scott's fellow Australian Cameron Smith shot two-under and compatriot Jason Day showed tremendous grit to fight his way to two-under while battling back pain that required treatment as he moved gingerly around the course. Another Aussie, Marc Leishman, shot an even 72.

A swashbuckling Woods crafted a remarkable recovery shot from the trees left of the 14th hole and drained a 25-foot birdie putt to join Harding (69) in a tie atop the leaderboard in Georgia before dropping a shot late in the day.

Woods, a four-time Masters champion, made the turn at one-under before hitting the green in two at the par-5 13th and two-putting for a birdie.

Scott was treading water at even par through 14 holes but surged up the leaderboard courtesy of birdies at the par-5 15th and difficult par-4 17th. He then made it three birdies in four holes when he picked up another shot on the 18th.

Day received treatment on his back beside the second hole and it was reported he aggravated his chronic injury by picking up one of his children in the morning, although that hasn't been confirmed. However, Day stood up from his early-round massage and belted a 278-yard second shot into the par-5 second and two-putted for birdie.

Speaking about Day's condition, Scott said: "He seems to have a bad back all the time and plays great. So I'm sure he knows what he's doing."

Smith bogeyed the first hole and also faltered on holes nine and 11, but bounced back with three birdies in the final six to finish two-under par.

Canadian Corey Conners was the bolter of the frontrunners, finishing at two-under after becoming the last man to book his Masters berth. Conners rolled in an eight-foot eagle putt at the par-five 15th on the way to a total of 70 in the first major of 2019. He would have found himself tied for the lead if not for a bogey on the final hole.

Conners only squeaked through qualifying into last week's Texas Open, then won it to punch his Masters ticket, and shook off a bogey at the third, getting back to even for the day with birdies at the 13th and 14th before his textbook eagle at 15, which briefly put him alone atop the leaderboard on three-under.

Meanwhile, Rory McIlroy, who can complete a career Grand Slam with a victory at Augusta, had a disappointing day, finishing one-over as more big names struggled to tame the famous course. Jordan Spieth was well back at three-over, as was England's Justin Rose.

American Phil Mickelson, who needs a US Open title to complete a career Grand Slam, was among those tipping McIlroy to contend for the trophy but the Irishman has a giant mountain to climb if he wants to add more silverware to his collection.

"He's had such a phenomenal start to the year, been playing with such great consistency week-in week-out, I think contending will be a given," Mickelson said. "You just need those little breaks, little things to happen that push you over the winner's circle."