Novak Djokovic stood stoically, as still as a stone -- was it another strong take on the mannequin challenge? -- and the smoke billowing at Londons 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, wouldnt be terribly surprising; he has won each of the past four editions of this tournament.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 Djokovics uneven 6-7 (10), 6-0, 6-2 victory over the 23-year-old Thiem, it would be a little too easy to declare Djokovics glorious run of dominance over. But its 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, arent particularly good. Lets just say that for Djokovic, the end of the season cant 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. Heres 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 youre always picking these kind of things.Q. If you keep doing these things ...Djokovic: I keep doing these things? Why dont you get suspended then?Q. You were close, werent you?Djokovic: Im close? Im still not suspended, so if Im not close, Im 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 didnt.Q: Youre not concerned about your mindset?Djokovic: Im the only player that shows his frustration on the court? Thats what you are saying?Q: Youre 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 Im in the process at the moment, and its 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 players 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 hasnt 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 Im 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 Im still here, and I feel like Im on the right path. Im in a better state of mind than I was some time ago. Thats all Im thinking about right now. Soccer Jerseys Outlet . -- Matt Kuchar and Harris English ran away with the Franklin Templeton Shootout, shooting a 14-under 58 on Sunday in the final-round scramble to break the tournament course record. 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