Novak Djokovic was full of praise for the "amazing" Denis Istomin after the Uzbek outsider pulled off an incredible upset by beating the defending champion in the second round of the Australian Open.

The 30-year-old wildcard produced the performance of his career at Rod Laver Arena on Thursday, winning a marathon five-set contest 7-6 (10-8) 5-7 2-6 7-6 (7-5) 6-4.

World number 117 Istomin had only taken one set off the 12-time grand slam champion in their previous five matches, but raised his game to topple the second seed.

Djokovic was magnanimous in defeat following an early exit which put paid to his bid to become the first man to win seven Australian Open singles titles.

The world number two said: "All the credit to Denis for playing amazing. He deserved to win. No doubt, he was a better player in the clutch moments.

"He stepped it up, played aggressive. Served very well, very precise. There's not much I could do.

"Of course, I was not pleased with my performance overall. But I have to congratulate my opponent."

Djokovic insisted he was not guilty of taking Istomin lightly.

He added: "There was intensity, of course. We played four and a half hours.

"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 that's sport.

"I never underestimate any opponent. Doesn't matter which tournament I play on or which round. I try to give the best that I can. I have respect for everybody.

"Him playing this well, it's amazing. He played obviously above his level. You got to give him credit for that. Many things came together for him. He's a well-deserved winner."