Why this is the most shocking loss of Novak Djokovic's career

— -- MELBOURNE, Australia -- The Day 4 schedule said Novak Djokovic was playing a journeyman pro from Uzbekistan in his second-round match at the Australian Open. But for a while now, Djokovic has been competing against the ghost of his past self, too. Over the past three years -- especially the first six months of 2016 -- he was the most ruthlessly efficient winner in tennis. Then Thursday's match started and, once again, Djokovic found himself struggling to reach his past standards -- this time absorbing the most shocking loss he has taken in years, at the Grand Slam tournament he has owned like no other.

"On any given day you can lose -- nothing is impossible," Djokovic said with a shrug after Denis Istomin, the 117th-ranked player in the world, outlasted him in a spectacularly played 7-6 (8), 5-7, 2-6, 7-6 (5), 6-4 win that took 4 hours, 48 minutes to complete.

How epic an upset was this? Istomin gained his way into this major only via a wild-card berth. Before this tournament, he had only $860 in earnings this year. That's it. At 30, he's one year older than Djokovic, but he was still bouncing between $50,000 Challenger tournaments and the ATP main tour, where he has won only one career title. In 2016, Istomin compiled an 8-22 overall record, while Djokovic was stockpiling the sort of accomplishments that moved him into the neighborhood of legends like Laver and Federer, Sampras and Nadal.

But Djokovic, a 12-time Grand Slam title winner, has had a rocky road since he won his first title at Roland Garros last June, a triumph that completed his career Slam. He sandwiched a third-round upset against Sam Querrey at Wimbledon and this mind-blowing loss to Istomin around a run to the US Open final, where he lost to Stan Wawrinka. He parted ways with coach Boris Becker in December after Andy Murray ended Djokovic's 122-week reign at No. 1 with a late-2016 closing rush. He has been working more publicly with Pepe Imaz, who runs a tennis academy in Spain that's based on the principles of peace, harmony, love and understanding the power of lengthy hugs.

In short, he's had a lot going on.

But none of Djokovic's other losses -- not even his first-round flameout at the Rio Olympics against Juan Martin del Potro that left him sobbing on court -- were as comprehensively damaging to the aura of invincibility like this seismic upset will be.

Because this wasn't a match that Djokovic handed to Istomin. He fought like hell. Tried everything. Insisted he didn't underestimate Istomin.

And Istomin went out and beat him anyway.

Istomin took the best shots Djokovic threw at him, especially in the fourth set, when Djokovic started blasting everything he had at him, trying to put his foot on the underdog's neck to avoid a fifth set, only to have Istomin handle the barrage, the pressure, the nerves, the cramping in his legs from the third set on -- all of that. Istomin was running down balls and smashing back winners of his own, many of them shots that hugged the side rails of the court and bounced just in beyond Djokovic's reach.

"All credit to Denis for playing amazing," Djokovic said. "He deserved to win. He was the better player in the clutch moments. He stepped it up."

Djokovic went on to explain more about why Istomin won, but he was not as forthcoming when repeatedly asked to pinpoint what has been lacking in him.

He mentioned some things about not "feeling" the ball so great Thursday, lacking a "rhythm," stuff like that. But he also insisted he felt perfectly fit. When asked if he is still experiencing a hangover from the French Open, he insisted no, no, he hadn't even thought about that win, let alone what has happened since. But he was unconvincing.

"It's not a time now to go deep into it," he finally said.

But if not now -- at a place he was trying to win his record seventh Australian Open title, breaking a tie with Rod Laver -- then when?

Djokovic is a very smart man. He leaves nothing to chance. You can be sure he will go home now and re-analyze everything about this past year and a half yet again.

For months now, he has tried to control the narrative about his dip since the French Open by insisting his results weren't that bad at all. Nothing to look at here! And that argument even got a little traction for a while, and some folks backed off. But what seems clearer now is Djokovic's fellow players -- the real constituency he might've been trying to speak to the whole time -- aren't buying the nothing-is-amiss story anymore. Not even the likes of Istomin.

And Djokovic knows it.

When asked if he believes the other players have come to believe he is beatable now, Djokovic calmly said, "Sure."

So what has changed these past seven months? The mental edge? In him? In them?

Once again, Djokovic didn't say yes. Not exactly.

"Make your conclusions," is all he said.