American Brooks Koepka has delivered the perfect answer to critics of his recent weight loss campaign, with a flawless opening round leaving him atop the leaderboard at the Masters.

The winner of three majors in the last 22 months, Koepka has dropped about 10kg in the last six months, although he has remained silent as to the exact reason.

It’s coincided with a dip in form, with the world No. 4 only managing two top-10 finishes from eight starts this year.

Weight isn’t the only thing Koepka has lost, he’s now about 10 metres shorter off the tee.

Earlier this week Golf Channel analyst Brandel Chamblee linked Koepka’s weight loss to rumours of a photo shoot for a magazine.

“For him to change his body and his body chemistry for vanity reasons, for a vanity shoot, is the most reckless self-sabotage that I have ever seen of an athlete in his prime,” Chamblee said.

“To do something that takes you out of your game, to change your game completely ... to see someone's body change drastically, it's never worked out very well.

“It's led to a deterioration.”

Koepka may be slimmer than ever before, but his shadow looms as large as ever, the 28-year-old reeling off five birdies in six holes starting at number 10, including four straight from 12-15.

Koepka makes third straight birdie

Speaking after the round, Koepka dismissed criticism of his weight loss.

“I lift too many weights, and I'm too big to play golf. And then when I lose weight, I’m too small,” Koepka complained.

“I’m going to make me happy. I don’t care what anyone else says.”

That was followed by a quick glance at the leaderboard in the press building, which showed him tied for the lead at -6 with Bryson DeChambeau.

"I'm doing it for me, and obviously it seems to work."

Koepka’s round of 66 was the highlight of an entertaining afternoon, that saw many of the best scores set late in the day.

DeChambeau could have snatched the lead right at the death, his second shot to the par four 18^th striking the flagstick and coming to rest just centimetres from the hole for a tap in birdie.

Other notables include Phil Mickelson, who shot 67 to sit in outright third, as he bids to become the oldest ever winner of a major championship, while Tiger Woods opened with a 70, the same score he’s shot in the first round in three of his four victories.

Tiger birdies the 14th

“I feel very good,” said Woods. “I played well today and I controlled my golf ball all day. I’ve shot this number and won so hopefully I can do it again.”

Former Masters champion Adam Scott is the best of the four Australians after an opening round of 69 that left him in a tie for 6^th. His round was highlighted by birdies at 17 and 18 that vaulted him onto the first page of the leaderboard.

It’s a solid start for the 38-year-old, who signed for his lowest opening round at the Masters in five years.

"Generally in my career, I'm a slow starter - certainly the last 10 years," Scott said.

"I don't jump out of the gates, it seems. So I don't know if this is jumping out of the gates, but it certainly is great."

Jason Day and Cameron Smith each shot 70, and Marc Leishman 72, although Day will be feeling less than happy after his ongoing back injury flared up before play.

Day leant down on the practice green to kiss his daughter and aggravated the injury, forcing him to seek treatment from the trainer on the second hole.

He was visibly limping during the latter stages of the round, making his score of -2 even more impressive, although questions must be raised about whether or not he can complete four rounds.

Day gets treatment on his back

At the other end of the scale world no. 1 Justin Rose and former champion Jordan Spieth endured a miserable day.

The pair both shot 40 on the first nine, and could only manage a single birdie on the way back to the clubhouse to each finish at +3.

It continues a difficult run for Spieth, who hasn’t won in nearly two years.

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