Boris Becker has questioned Novak Djokovic's desire and fight following his shock second-round defeat to Denis Istomin at the Australian Open today.

Djokovic, a six-time winner in Melbourne, was beaten in five sets by the world number 117 to suffer his earliest Grand Slam exit since Wimbledon in 2008.

Becker, who is a two-time Australian Open champion himself and coached Djokovic until the pair split at the end of last year, believes that the Serb was too "passive" and "defensive" against the Uzbek wildcard.

"I'm really shocked that Novak Djokovic lost. I thought he was playing much too defensive and never really took the initiative and never really fought for it. I absolutely never expected him to have trouble with Denis Istomin. No disrespect but Nole is a six-time Australian Open champion," he told Eurosport.

"But I could tell in the first set he was way too defensive and way too passive. The first set in a three-out-of-five match is always very important. In the fourth set again he was a bit lethargic and not playing at full power. Then once you're in the fifth set anything can happen and credit to Denis, he kept his composure, he kept his nerve, he went a break up and kept serving well and hitting the lines so it's well deserved.

"It's a tough one - I feel for him and I still have so much respect for him. This is a new situation for him. The Australian Open was always the foundation of the year and you leave Australia with the title in the bag and the 2,000 points and the year plays much easier. And now it's a new situation and I think the team has to stick together and sleep on it because nothing is going to come out of it tonight. They need to find a cure for this current situation."

Djokovic was looking for a record seventh title in Melbourne and has lifted the trophy in four of the last five editions of the tournament.