Novak Djokovic stood stoically, as still as a stone -- was it another strong take on the mannequin challenge? -- and the smoke billowing at London's O2 Arena began to engulf him.

Before his opening match at the Barclays ATP World Tour Finals against Dominic Thiem on Sunday, Djokovic stood there and briefly closed his eyes, and you have to wonder whether, even for a moment, he wished he could make himself disappear.

For four of the past five seasons, Djokovic has finished as the No. 1-ranked player. He could still make it five out of six with a victory in this eight-man event, something that, based strictly on history, wouldn't be terribly surprising; he has won each of the past four editions of this tournament.

It took Novak Djokovic a while before he found his groove at Sunday's ATP World Tour Finals. Clive Brunskill/Getty Images

After losing the No. 1 last week to Andy Murray, the world No. 2 essentially needs to go one step further than Murray in London.

With Djokovic's uneven 6-7 (10), 6-0, 6-2 victory over the 23-year-old Thiem, it would be a little too easy to declare Djokovic's glorious run of dominance over. But it's not irresponsible to wonder whether his shelf life at the top has a rapidly advancing expiration date.

The recent signs, including this manic match with Thiem, aren't particularly good. Let's just say that for Djokovic, the end of the season can't come fast enough.

The frustrations were still evident Sunday, even after a smooth final two sets. In his postmatch news conference, Djokovic had a heated exchange with reporters when the world No. 2 found himself on the defensive answering questions about his recent bouts of temper. Here's how it went down:

Question: Back to the end of the first set, we saw you similarly venting your frustration at Roland Garros, throwing your racket. Does it concern you one day that will cost you dearly? If it hit someone ...

Djokovic: You guys are unbelievable.

Q: Why is it unbelievable?

Djokovic: Because you're always picking these kind of things.

Q. If you keep doing these things ...

Djokovic: I keep doing these things? Why don't you get suspended then?

Q. You were close, weren't you?

Djokovic: I'm close? I'm still not suspended, so if I'm not close, I'm not close.

Q: If that ball had hit a spectator, it could have been serious.

Djokovic: It could have been, yes. It could have snowed in O2 arena, as well, but it didn't.

Q: You're not concerned about your mindset?

Djokovic: I'm the only player that shows his frustration on the court? That's what you are saying?

Q: You're one of the top-ranked players in the world.

Djokovic: So?

Just five months ago, Djokovic seemed invincible. He won the Australian Open, the rare Indian Wells-Miami double and then the French Open, completing his career Grand Slam. That was his 12th major singles title, and that noise you heard was the crunching of all-time numbers.

How long before the 29-year-old Serb caught his long-time rivals, Roger Federer (17) and Rafael Nadal (14)? Since taking three Grand Slams in 2011, Djokovic had won an astounding 11 of 22 majors up to that point. That trajectory would vault him past Nadal -- and Pete Sampras, too -- by the end of 2017. Federer would be in his sights by 2018.

And then Djokovic -- perhaps in retrospect it was inevitable -- came back to the field. He lost to Sam Querrey in the third round at Wimbledon, Juan Martin del Potro in his first match at the Rio Olympics and Stan Wawrinka in the US Open final.

After falling to Marin Cilic in the quarterfinals of the BNP Paribas Masters earlier this month, Djokovic unblinkingly tackled the elephant in the room: his disturbing and dramatic loss of confidence on the court.

"A lot of things, a lot of emotions went through my mind and body in the last two years, Djokovic said, "with the things that I was blessed to achieve, obviously, and the career Grand Slam this year. But it took a lot out of me, and it has put some things in perspective and, obviously, raised some questions in which direction I want to go to.

"So I'm in the process at the moment, and it's going [to] obviously take some time really for me to redefine all these things."

Against Thiem, it took a 69-minute first set to accomplish that reset.

Djokovic was tentative in that frame and Thiem was consistently fiery. Then, after earning three set points in the tiebreaker, Thiem succumbed to nerves, pounding back-to-back double faults and a tight backhand into the net. Finally, on his seventh set point, he whipped a forehand winner.

At one point, Djokovic lost his racket and his legendary balance, winding up on his behind, then offered a thin smile -- maybe it was actually an embarrassed grimace.

Whatever it was, it returned him swiftly to full power.

As Federer and Nadal have both learned, the 30th year of a professional tennis player's life can be a difficult one. After winning his 16th major, at the 2010 Australian Open at the age of 28, Federer has won only one since. Nadal had just turned 28 when he won the 2014 French Open -- and he hasn't won one since. Sampras, too, won one major after turning 29.

Djokovic celebrated his 29th birthday just before the French Open. He has struggled with physical and mental issues ever since winning that cherished, life-changing title.

This season-ending tournament offers a revealing glimpse into his psyche.

"Look, in the end of the day, I have to get to that state of mind where I'm able to perform as well as I want to match after match," Djokovic said at the Paris Masters. "I was not able to find that level for last couple of months.

"But I'm still here, and I feel like I'm on the right path. I'm in a better state of mind than I was some time ago. That's all I'm thinking about right now."