Novak Djokovic has seemed like a man often having an argument with himself since he was unexpectedly knocked out in the third round of Wimbledon in July. Once an uncomplicated, all-business competitor, a less motivated, more philosophical Djokovic has emerged in recent months.Signs of this tension in Djokovic were on display Tuesday evening at the ATP World Tour Finals in Londons O2 arena in what was the closest thing he has ever had to a must-win round-robin match. The familiar, fierce competitor ultimately prevailed as Djokovic punched through to the semifinals with a 7-6 (6), 7-6 (5) win against Wimbledon finalist Milos Raonic, against whom he is now 9-0.But it wasnt without signs of his ongoing interior struggle.If I had to pick one thing [to explain why I won], I would say it was my just hanging in there mentally, waiting for my opportunity to come, Djokovic said in his on-court interview after the match. Milos was playing very well. It was a very close match, just a few points separated us.True, but ...How often did the dominant Djokovic of recent memory allow an opponent who has never beaten him reach set point after Djokovic had forged ahead by a break?There were dangerous lapses against Raonic, who is capable of taking an opponents racket out of his hand with the sheer efficiency of his monstrous serve. Just as significant, Raonic led by a mini-break in each of the tiebreakers. His wounds from that point on were largely self-inflicted. Put them down to an understandable lack of confidence in key moments.Djokovic has probably dallied in like fashion before, but rarely at this event and in this venue, where he has routinely been at his zenith. He has won 20 of his past 21 matches at the O2 and captured the title four years in a row.All that raises a question: Why would anyone mindful of Djokovics 23-0 record against the other three men in his group call this round-robin engagement a must-win match?For one, Djokovic was recently stripped of his No. 1 ranking by Andy Murray. If Djokovic hopes to get it back and finish the year ranked No. 1, he will need to win one more match than Murray. Murray won his opener and plays Kei Nishikori on Wednesday (9 a.m. ET on ESPN3).?Heres another reason: Despite that 0-9 record against Djokovic, Raonic might be the best player who has yet to win a major. Hes hungry, dedicated and on the cusp of a breakthrough prefigured when he eliminated Roger Federer in the Wimbledon semifinals.But perhaps most important, Djokovic is under pressure to show hes not at war with himself. His identity as a hard-edged tennis champion is in conflict with self-absorbed concerns about his evolution as a person and the peace-and-love philosophy espoused by his Svengali-like adviser, Pepe Imaz.Things have gotten complicated for Djokovic in recent months. His nerves have showed signs of fraying. He has had a year that qualifies as spectacular, with two Grand Slam titles and a career Grand Slam in the books, yet his struggles after Wimbledon have been conspicuous.If he wins this event for a fifth consecutive time and retakes the top ranking, the losses in his past four outings will pale in significance. The crushing defeats inflicted by Juan Martin del Potro in the Olympic Games and Stan Wawrinka in the US Open final will be put down to his minor injuries.It was a very close match, Djokovic told reporters after beating Raonic. It really could have gone either way.It was a close match against Raonic, one that could have gone either way. 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