Former coach Boris Becker said Novak Djokovic was unrecognisable mentally as the fallout from the defending champion's shock Australian Open exit continued.

A six-time winner and two-time reigning champion, Djokovic was sensationally bundled in the second round 7-6 (10-8) 5-7 2-6 7-6 (7-5) 6-4 by wildcard Denis Istomin after a five-set marathon on Thursday.

Djokovic was below his best against the world number 117 as he made his earliest grand slam exit since Wimbledon in 2008.

Dissecting Djokovic's remarkable elimination, Becker said the Serbian former world number one lacked intensity and the absolute will to win.

"I didn't recognize him, his mentality," Becker told the New York Times.

"It's a big door open now. We talked about the next gen for a long time, and now is the time for these 19-, 20-, 21-year-olds to go through the door.

"When the top dog is struggling a bit — and no disrespect to Andy, but Novak was the top dog — the way was blocked. But now it's a shift."

Becker and Djokovic parted ways in December after three years and six grand-slam titles.

The split came as 12-time major winner Djokovic was dethroned by Andy Murray atop the ATP rankings, having flopped at Wimbledon and the Olympic Games in Rio de Janeiro before losing to Stan Wawrinka in the US Open final.

"Obviously the second half of last year, there was a different priority," Becker added. "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.

"Having watched the match, 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 continued: "No shame in losing. You play against Murray, [Rafael] Nadal or Wawrinka in the final of the Open, you can lose to those guys. Even Novak playing good can lose. But not against Denis Istomin. Novak wasn't injured, apparently. He was two-sets-to-one up. That is unusual and doesn't fit into the picture I have of him."