Why so serious?

NEW YORK – Novak Djokovic has devoted his life to channeling positive energy. He does yoga. He meditates. He shuns cold water because it diverts blood from his muscles. A natural showman, he jokes and jabs and feeds off the spotlight.

This year, the top-ranked Serb and U.S. Open favorite, who often sounds like a self-help guru, has talked about how marriage and fatherhood in 2014 have brought holistic balance to his life. But so far at the U.S. Open, his mood has been downright cranky.

He’s disparaged his play, calling it “sloppy” and “frustrating.” He’s groused about tepid crowd support. He’s angrily removed clothing, screamed at his box, smashed rackets and incurred umpire warnings.

On Sunday, after dropping the second set during a tough 6-3, 4-6, 6-4, 6-3 defeat of Roberto Bautista Agut in the round of 16, Djokovic went an ornery step further. He walked to the changeover chair and stomped on an innocent stick poking out of his on-court bag.

Brad Gilbert, the ESPN analyst, tried to gag with Djokovic in the on-court interview about the broken racket and subsequent warning, but the Serb dismissed his line of questioning. Gilbert moved on.

“He’s normally a little more jovial,” said Gilbert.

For all the talk of Serena Williams’ quest for the Grand Slam, Djokovic’s year has been nearly as spectacular. He has more wins (61-5 to 53-2) and more titles (6 to 5) than Williams. Were it not for a sizzling hot Stan Wawrinka in the French Open final, Djokovic would also be seeking a calendar year Grand Slam after sweeping majors at the Australian Open and Wimbledon. Williams is 26-0 in Slams this season; Djokovic is 25-1.

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Djokovic, 28, also has been by far the ATP Tour’s best hard-court player for the past five years. He’s captured four Australian Opens on cement in that period, and five overall. On hard courts since 2011, he is 227-25 with 26 tournament wins — incredibly, more titles than losses. But despite reaching his ninth consecutive U.S. Open semifinal by defeating Feliciano Lopez of Spain in four sets on Tuesday, he has only one trophy to show for it, in 2011.

A popular theory is that his irascibility is due to the high stakes. Michael Chang, the 1989 French Open champion, said Djokovic is more tightly wound at the majors than lesser events generally. “Part of it is that he feels pressure and he wants to do well,” said Chang, who coaches 2014 U.S. Open runner-up Kei Nishikori of Japan. “There is more on the line.”

Another idea is that the Serb, who hasn’t played his best since winning a third Wimbledon in July, is anxious about his level. Before the U.S. Open, he lost to Andy Murray and Roger Federer in the finals of lead-up events at Montreal and Cincinnati. He even appeared irritated by the surprise ambush attack Federer pulled on him in the Cincinnati final. Federer deflected his second serve almost as a half volley and rushed the net.

Todd Martin, a 1999 U.S. Open finalist who co-coached Djokovic for several months in 2009-10, said the Serb is susceptible to ornery outbursts, but less so than his younger days. Djokovic has become a master at understanding and managing his moods. But he agreed that a U.S. Open win would take “a really good year and make it one that goes down for the ages.”

Martin thinks that Djokovic’s behavior might be rooted in an unconscious motive: steeling himself for a possible clash with No. 2 Federer in the final. The Swiss is a fan-favorite wherever he goes and enjoys almost universal home-court advantage. “Maybe subconsciously it’s almost a preparation,” said Martin.

Players, of course, deal with on-court emotions in different ways. John McEnroe, labeled “Super Brat” by the British press in his cantankerous heyday, would explode – and play better. Jimmy Connors whipped crowds into a frenzy and elevated his game. Bjorn Borg and Ivan Lendl betrayed few feelings at all. Federer operates on a wave of silky smooth joy. “I mean, I’m not the kind of guy who wakes up angry,” 17-time major winner Federer laughed after dispatching Richard Gasquet of France in the quarterfinals Wednesday.

Although nine-time Grand Slam champion Djokovic is no longer the court jester who entertained fans with spot-on player impersonations, he has always been an emotional competitor. After outlasting Wawrinka 12-10 in the fifth set in a 2013 Australian Open quarterfinal slug fest, Djokovic Hulk-ripped his shirt in half. He likewise has shown an ability to use controversy as motivational fuel and play with his back against the wall.  He has triumphed in hostile Davis Cup settings and led Serbia to its sole team title in 2010. Twice in the U.S. Open semifinals he staved off match points against Federer to reach the final.

“The last thing that’s good for him is to suppress emotion,” said Martin. “When he’s really calm, it’s not because he’s emotionless. It’s because he’s at peace. And he still lets the positive stuff show. It’s flushes right through the system. There’s a non-coagulant quality to it.”

But Djokovic doesn’t always play the New York crowd right. In 2008, stoked by quotes from Andy Roddick about his onetime penchant for injuries and retirements, Djokovic lashed out during the on-court interview after beating Roddick in the U.S. Open quarterfinals.

“You know, Andy was saying I have 16 injuries in the last match,” Djokovic said defiantly. “Obviously, I don’t — right?” Boos rained down from spectators at Arthur Ashe Stadium.

Against Bautista Agut this week, Djokovic again wasn’t feeling the love. He put his finger to his ear to solicit the crowd’s support when he broke the Spaniard for a 3-2 lead in the fourth set.

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Later, an irritated Djokovic said: “I understand also the crowd that likes to support the underdog. They want to see a good match. But, you know, it’s on you to focus on the support that you have. At the end of the day you’re alone on the court. You got to do your job regardless of who is cheering you on or not.”

The Czechoslovakian-born Martina Navratilova said it wasn’t until she was on the downside of her career that the New York fans warmed up to her. It didn’t help that her main rival, Chris Evert, was dubbed “America’s Sweetheart.”

“I can relate to that because I was in that boat a long time ago,” said the 18-time Grand Slam champion, who became a U.S. citizen in 1981.

Navratilova, who wore her emotions on her sleeve, said pandering too much to New York fans could backfire. “It can come off as whiny,” she said. But a cranky Djokovic could win the title. She expects him to tone it down now that he is within sniffing distance of the championship. “He needs to calm down a little bit because otherwise it’s too much energy wasted,” Navratilova said.

Despite his sour mood, Djokovic is where he wants to be: Six sets from the hardware. In Friday’s semifinals, he meets last year’s champion, No. 9 seed Marin Cilic of Croatia. Federer faces No. 5 seed Wawrinka in the other all-Swiss semifinal.

Djokovic is 13-0 against the 6-foot-6 Cilic, including a straight-set defeat in the Wimbledon quarterfinals. He will be a big favorite. He knows he can play better than he has so far.

His mood may not matter. And maybe there’s a simpler explanation: lack of sleep.

“Maybe it’s because of kids,” laughed Chang, who has two young daughters.

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