While Day is undoubtedly in the box seat heading into the weekend, Herbert, who famously slept in the Carnarvon Golf Club auto park during Open qualifying three years ago then led after the first round last year at Royal Sydney, collected six birdies before dropping his only shot of the day at the last.
"I didn't know where I was, to be honest, after the third round", Davis said of his six-stroke deficit.
Cameron Smith also shot a 68 to finish fourth on nine-under but none of them was able to mount a challenge to knock Davis off the lead as the Sydney crosswinds wrought their customary havoc. "I'm going to remember this forever".
"You can't be ideal all the time", Day said.
"It's my first major so I've got no idea what to expect".
"Even though I had a great round, I didn't think I was going to be far enough up the leaderboard". But he missed a 15-foot putt on the last to force a playoff.
With five leading groups still playing when he signed for his 11-under 273, Davis was shocked that he had ended up with the Stonehaven Cup in his hands, A$225,000 ($171,270.00) in the bank and a spot at next year's British Open.
"I'd love to explore it", Spieth said.
Davis, without a previous top-10 result since turning professional a year ago and who missed the cut at last week's NSW Open, is the most unlikeliest champion since Aaron Baddeley beat Greg Norman down the stretch as a teenager amateur in 1999.
But he has been struggling to make his mark while playing on the Canadian PGA Tour.
Spieth never seemed comfortable at The Australian Golf Club earlier in the tournament and couldn't break 70 over the first three rounds, posting earlier scores of 70-71-70 for a tournament total of 6-under 278.
The 24-year-old completed the unsuccessful defense of the title he won in 2014 and again last year with a flourish, though, when he finally found his range with the putter and nailed a eight-footer for an eagle. That's going to be pretty tough to do, but I'm going to give it my best shot now that I've actually got the opportunity to go and play in it.
"I was trying to take my mind off that (but) I'm sure later on today I'm going to start looking around and seeing what this might get me".
Defending Australian Open champion Jordan Spieth eagled his final hole to end a disappointing week on a high and finish eighth at six under.
"It's a little bit numb at the moment", said the 22-year-old Australian, as reported by ESPN.
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