Tennis legend Andre Agassi on Saturday backed misfiring Novak Djokovic to pull out of his slump after he crashed out of the Australian Open.

Describing Djokovic as "one of the greats", Agassi said the Serb could soon be back to his best, comparing the former world No 1's nosedive to his own troubles in the 1990s.

Djokovic, 29 and seeking a record seventh Australian Open title, lost to 117th-ranked Denis Istomin of Uzbekistan in the second round.

"What I can speak to is Novak's game and his abilities and there's no reason in the world why he can't turn it around," Agassi said.

"Novak won't need to learn in as hard a way as I had to learn. He still has time, and he's a heck of an athlete.

"In my opinion he is one of the greats of all time, so I give him a high chance coming back in a hurry."

Agassi battled back from injury, marital problems and, as he later disclosed, drug problems to relaunch his career in the late 1990s, winning five of his eight Grand Slam titles.

Djokovic's return to the top could face an obstacle in Andy Murray, who has replaced him as No 1 and, according to Agassi, could now get even better.

"He's very impressive. I've always talked about Andy as a player that has never really realized his maximum potential," Agassi said.

"He's so good at certain things it almost makes him a bit indecisive. It almost makes him not bring his strengths to the table as much as he could.

"If he actually minimized his defensive skills by just five percent he might be a better player.

"My expectation is he is only going to get better. It looks like he's strong. It looks like he's fit. It looks like he knows how to invest in himself."

Agassi also had well-wishes for troubled Aussie Nick Kyrgios, who was booed off the court and accused of not trying in a perplexing five-set defeat to Italy's Andreas Seppi in round two.

"He's obviously a rebel of sorts. He's a fighter of sorts. He might choose to take some of those fights and fight himself," Agassi said.

"I would much prefer to see him fighting his opponent and fighting for the opportunity to get better and to do some of the things he's capable of.

"I wish good for him because he obviously isn't comfortable in his own skin at the moment."

Mom hard to please

Meanwhile, giant-killer Istomin said his mother still wasn't too excited even after he followed his stunning upset of Djokovic with another five-set victory on Saturday.

The world No 117 sent shockwaves through the tournament by ousting six-time champion Djokovic, but he isn't done yet as he beat 30th seed Pablo Carreno Busta 6-4, 4-6, 6-4, 4-6, 6-2.

Known for his colorful glasses, the 30-year-old Istomin is now into the fourth round for the first time and will play Grigor Dimitrov.

But he said his mother, Klaudiya, who also serves as his coach, was hardly jumping for joy.

"She said, 'Great job'," Istomin smiled, when asked to describe her reaction, adding that he might have to lift the trophy on Jan 29 get his mom excited.

"Maybe if I win the Australian Open, maybe she will ... but I'm not positive about that," he said.