Novak Djokovic has described his abrupt Australian Open exit as just 'one of those days'.

The defending champion was hunting for his seventh title in Melbourne following his triumph last year, but he failed to make it past the second round.

The Serbian, who lost his world number one ranking to Andy Murray at the end of last season, was beaten 7-6(8) 5-7 2-6 7-6(5) 6-4 by Uzbek wildcard Denis Istomin in a gruelling match that lasted just under five hours.

"There was intensity, of course," Djokovic told reporters. "We played four-and-a-half hours. It's just that, you know, it's one of these days when you don't feel that great on the court, don't have much rhythm, and the player you're playing against is feeling the ball very well.

"So, you know, that's sport. I started the season very well. Again, it's a tennis match. On a given day, you can lose. I mean, nothing is impossible. What can I do? I did try my best till the last shot, but it didn't work."

Djokovic's exit boosts the chances of Murray, Roger Federer and Rafael Nadal, who all remain in the tournament Down Under.