Novak Djokovic's ex-coach Boris Becker has slammed his the former world No.1 following his shock loss to world No.117, Dennis Istomin.

Djokovic, the defending champion and six-time Australian Open winner, lost a five-set epic, 7-6 (10-8) 5-7 2-6 7-6 (7-5) 6-4, to the 30-year-old Istomin.

It is the first time Djokovic has lost prior to to third round in in his last 34 Slams.

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

"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."

Istomin will now face 30th seeded Spaniard, Pablo Carreno Busta, in the third round.