Novak Djokovic's former coach says tennis is no longer Djokovic's top priority, and there's a key ingredient missing from his game

Novak Djokovic's recent slump hit a new low in the Australian Open after he lost in five sets to 117th-ranked Denis Isotomin in the second round.

The loss was Djokovic's earliest exit in a major since 2008 and considered, by some, the "most shocking loss" of his career.

For Djokovic, who lost the world No. 1 ranking in November, it raises more questions about why he is suddenly struggling.

Djokovic's former coach, legendary tennis player Boris Becker, told The New York Times' Christopher Clarey that off-court issues may be plaguing him. Becker split with Djokovic last year and told Clarey that he thought tennis was no longer Djokovic's top priority.

"Obviously the second half of last year, there was a different priority. Novak was the first one to admit that, and I think that was the main reason for me to stop this because I thought my job isn't that important anymore obviously."

According to Clarey, after the U.S. Open last year, Djokovic referenced "private issues" weighing on him.

Whatever Djokovic may be struggling with off the court, Becker believe it's messing with his competitive mentality.

"Having watched the match today, I felt he tried and he played five sets and four and a half hours, but I didn't see the intensity, didn't see the absolute will to win, didn't see him mentally going crazy.

"He always was very nonchalant about it, and that is not the Novak that I know. I'd rather see him break a racket or pull the shirt or something, for him to get emotional. I thought it was very even keel the whole match through, and that was unusual, and I don't know what to make of that."

Becker also referred to Djokovic as playing with "the hand brake on."

Of course, part of Djokovic's loss was the astonishing play and grit of Isotomin, who Djokovic noted after the match was a "well-deserved winner."

However, for a player who was beginning to look unbeatable, Djokovic's sudden decline is raising questions about his health and his mental state. Becker told Clarey that he hopes Djokovic "goes back to the drawing board and remembers what made him good in the first place." However, as Becker suggests, until tennis is Djokovic's top priority, the struggles may continue.