Novak Djokovic lacked fight and played too defensively in his shock Australian Open second-round defeat to Denis Istomin, says his former coach Boris Becker.

The two-time defending champion was stunned by the world number 117, who triumphed 7-6 (10-8) 5-7 2-6 7-6 (7-5) 6-4 in a marathon encounter on Rod Laver Arena.

Djokovic endured a disappointing second half to 2016, as he flopped at Wimbledon and the Olympics before losing the US Open final to Stan Wawrinka en route to being usurped as world number one by Andy Murray.

Victory over Murray in the Qatar Open final in Doha this month suggested a fresh start for Djokovic, but his loss was the earliest he has exited a grand slam since Wimbledon in 2008.

And Becker, who ceased working the Serbian at the end of last year, questioned Djokovic's desire against Istomin.

"I'm really shocked that Novak Djokovic lost," Becker told Eurosport.

"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.

"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."

Becker urged Djokovc to dig deep in order to rediscover his best form.

"It's a tough one – I feel for him and I still have so much respect for him," Becker said.

"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 2000 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. They need to find a cure for this current situation."