Perhaps it was the euphoria of bowling India out for 36. Perhaps it was the false impression created by a quick and comfortable fourth-innings chase of fewer than 100 to win that same sunny Adelaide afternoon. Or, perhaps, it was the confidence built up by last summer's clean sweep of Pakistan and New Zealand, a confidence that looks increasingly misplaced.
Australia entered Boxing Day at the MCG with very little sense of foreboding about what might occur should they bat first on another pitch that featured a liberal covering of grass to ensure it would not be too hostile to bowlers.
In fact, Australia were so confident that Joe Burns' second-innings 50 at Adelaide Oval had righted the numerous wrongs of the first innings, that Steven Smith's rapid demise at the hands of R Ashwin was a blip, and that Matthew Wade, Marnus Labuschagne, Travis Head and Cameron Green were all set for big innings, that the captain Tim Paine chose very happily to bat first on an MCG pitch that had 11mm of grass and early morning moisture.
In fairness to Paine, there was plenty of history backing this decision. Since the dramatic first day of the 2010 Ashes Test in Melbourne, when England sent Ricky Ponting's team in and promptly razed them for 98 to set up the retention of the urn, the average first-innings score was in the region of 389: more than enough, one would think, against an Indian side now minus Virat Kohli.
But the evidence presented by Australia's top six in front of a socially distanced MCG crowd of 27,615 offered rather more unsettling conclusions for Australia's planners and selectors. Confronted, for the second time in as many Tests, with a sensibly marshalled bowling attack on a pitch that required hard graft rather than heavy hitting, the Australians looked little better in a technical or tactical sense than they had done during the uncertain summer of 2018-19.
That this could be true with Smith present again was still more worrying for the hosts, who are now faced with the fact that, apart from his pair of stirring SCG centuries in the opening ODIs, Smith is playing much more like he did during a halting IPL campaign than at any stage of his otherworldly 2019 Ashes series in England.
Believe it or not, Smith is now closing in on three full years without making a century in a Test match in Australia, the sort of figure that many would refuse to believe without actually consulting the calendar. Last summer, New Zealand constrained his scoring rate with short stuff from Neil Wagner; this time around, the wily Ashwin is continuing to build on some early uncertainties created when they crossed paths in the aforementioned IPL.
By playing around neatly with lengths and paces on a highly disciplined, even slightly defensive, line of middle and off stumps, Ashwin has found Smith's outside and inside edged in consecutive innings for the combined tally of just one run. He found enough purchase in Melbourne to achieve similar things against Paine, after Wade had gifted his wicket to Ashwin with an unsightly smear before lunch - the sort of shot selection that no career opener would have sat comfortably with.
That Wade would have sallied forth in such a manner indicated a measure of overconfidence, a sense that may have arrived through the contrast in how he handled the opening exchanges when lined up against Burns, who groped, wafted and strained at Jasprit Bumrah over the course of ten torturous deliveries.
Burns had, at least, survived more than the single ball he managed against Trent Boult this day last year, but it was clear that the problems he has experienced so far this summer at all levels were not to be eradicated by a fourth innings cameo against a crestfallen India in Adelaide, after the game had been effectively decided.
For a time, the best hope Australia had of a substantial first-innings score was carried through by Labuschagne and Head, who in a stand worth 86 vital runs either side of the lunch break demonstrated that a good degree of application, with the odd aggressive stroke thrown in, could bring about the results Australia desired.
During this period, Rahane came close to looking like he was short of ideas, particularly after Bumrah was not called upon until midway through the afternoon session for reasons that were not entirely clear. Labuschagne left as many balls as he could, often on length, and was twice fortunate to have lbw shouts rebuked by ball-tracing on the grounds of height.
His back leg will show a bruise or three from balls that thudded into it with the bat clearly raised, but the proof of Labuschagne's judgment is in the fact he has got closer to a major score in each first innings than any other member of the home side's top six.
Contrast this with Head, who while playing soundly for the most part remains keener than most Test batsmen to feel the thud of the ball on the bat. Head leaves only around 15% of deliveries bowled to him, as against 29% for Labuschagne and 24% for Smith. It's a set of numbers that could not be forgotten when, after his post-lunch sabbatical, Bumrah angled in from around the wicket to coax an edge and the breakthrough. Head's average against balls whirring in at him from this point of release is around the 25-mark, and it was a surprise India did not opt for it sooner.
Labuschagne's handy occupation, and a shorter one from the sophomore Cameron Green, were then to be ended by the spiky, speedy work of the 26-year-old Mohammed Siraj, who deputised grandly for Mohammed Shami with spells of pace and direction. Labuschagne leaned too far across his stumps to avoid flicking a straight ball to leg gully - for once mimicking Smith in a fashion he would rather have avoided - and Green's immobile front leg presented Siraj with too clear a target for an lbw verdict. And 124 for 3 quickly became 155 for 7, the advantage very much lost.
One of the features of this match are a series of tributes for the late, great Dean Jones. His wife and daughters were accompanied to the middle by Allan Border during the tea break to place Jones' baggy green cap, Kookaburra bat and groundbreaking sunglasses by the stumps. Both Jones and Border were part of one of Australia's least happy Boxing Days of all, when they were bowled out by England for 141 in 1986 to set up an innings defeat. Undue haste had, at times, been a feature on both that day and this one.
Watching all this, the coach Justin Langer would have ruefully recalled his pre-match words, which featured plenty of confidence but also included the truism of Test match first innings: big ones win games consistently, and anything else will leave a side scrambling for freakish things like the third afternoon in Adelaide.
"If we're going to become a great team we have to get better at winning after we win and people didn't quite understand that, but really good teams keep winning and winning, particularly when they're playing good cricket," Langer said. "So it's an area we've addressed, we'll have to start well Boxing Day morning and then be consistent, because we know India will fight back as we saw in the first two days of the Test match in Adelaide.
"We know that in first innings in Australia we are looking to score 400 in the first innings - there is no surprise there, that's what we've based our best Test cricket on for years. So, when I said we have got areas where we can improve, that's one I am talking about. We play our best cricket, as we saw all last summer, when we are scoring big first-innings totals, that's what we aspire to and what we will be aspiring too in this game as well."
But having been fortunate to watch everything click for the pacemen in Adelaide at precisely the right moment, the Australians were only good enough to improve on their halting first innings of the series by the measly matter of four runs. Asking any bowling attack, even one as good as Mitchell Starc, Pat Cummins, Josh Hazlewood and Nathan Lyon, to pull consecutive Tests out of the mire is more than any international top order should expect.
India, as it happened, ended the day on 36 again. This time, though, for the loss of just one wicket. Reality was starting to catch up.
Daniel Brettig is an assistant editor at ESPNcricinfo. @danbrettig