The strapping Koepka revealed in March he had dropped about 10kg since November, though he was coy about the reason. He also lost some edge from his game, with just one top-20 finish so far this year.

Having retained the US Open title and then won US PGA Championship in a stellar 2018, the 28-year-old would normally be among the hottest favourites at Augusta National, but that's not the case.

Golf Channel analyst Brandel Chamblee on Wednesday linked Koepka's form to rumours of a photo shoot for ESPN magazine's The Body Issue.

"We know what (Koepka) did at the end of last year. You could extrapolate what he did at the end of last year to what he was likely to do here at Augusta National if everything was the same," Chamblee said.

"Now 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.

"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 was 33rd on his Masters debut in 2015 before finishing tied 21st in 2016, tied 11th in 2017 and missing last year's Masters with a wrist injury.

Koepka said he had returned from that injury-enforced lay-off with a greater appreciation for golf.

He certainly wasn't talking like a player unprepared to contend for the green jacket.

"I've never been a major champion when I've played here before," Koepka said.

"I'm a completely different player, probably. I understand that I handle pressure a lot better. I understand this golf course a lot better.

"Even sitting out a year, there's certain things you can pick up watching and I've kind of matured on and off the golf course.

"That makes a big deal."

Australian Associated Press