Novak Djokovic has failed to successfully defend his Australian Open title after getting stunned by world number 117 Denis Istomin in the second round.

The Serbian has struggled for consistency since his shock early exit from Wimbledon last year, and he faces a battle to take back his number one world ranking from Andy Murray following his latest result in Melbourne.

Istomin, who entered the tournament as a wildcard, managed to see off the 12-time Grand Slam champion 7-6(8) 5-7 2-6 7-6(5) 6-4, but it took him a draining five hours to claim his first win in six over Djokovic.

The Uzbek player showed his intent early on by creating six break points on Djokovic's opening serve, but he failed to convert a single one.

After trading breaks, the first set was forced into a tie-break, which Istomin took 10-8, giving himself the lead after one hour and 28 minutes.

Djokovic recovered in the second set, despite missing an early break point, but he did enough to get over the line, and in the third he took just over 40 minutes to put himself in the driving seat.

The 29-year-old's momentum did not last long, though, as another tie-break was needed to separate the pair, but rather than wrapping up the match, Djokovic relinquished the breaker 7-5.

The deciding set was surprisingly comfortable for Istomin as he nabbed that all-important break point, while Djokovic failed to test his opponent's serve.

Spaniard Pablo Carreno Busta awaits Istomin in the third round.