Reigning Australian Open champion Naomi Osaka has been knocked out of this year's tournament in the third round by Amanda Anisimova.

The 24-year-old's defence of her crown ends in the third round following a thrilling three-set battle with the American, who prevailed 4-6 6-3 7-6 (10-5) at the Margaret Court Arena.

Osaka won her first break in the opening game of the match and threatened to do so again in the third, but the world number 60 rallied to reduce the deficit to 2-1.

Aminisova failed to capitalise on a break point in the sixth game as Osaka stormed into a 4-2 lead, and the American's frustration nearly boiled over as the defending champion took the first set.

Osaka's serve was finally broken by Aminisova in game four of the second set as the 20-year-old moved into a 3-1 lead, and the American levelled the scores at 1-1 after Osaka smashed a forehand into the net to set up a deciding third set.

An array of unforced errors from Osaka enabled Anisimova to level the third set at 5-5 with an ace, with the underdog producing a stellar display in the super tie break to end Osaka's title defence.

"I'm speechless. I can't stop smiling. I'm just laughing. I absolutely love this," Anisimova said during her on-court interview.

"Going into this match I knew I had to be playing sharp if I wanted to give myself a chance. She [Osaka] is an absolute champion so I knew I would have to step up my game and be aggressive.

"I'm honestly so grateful that I was able to play so well today and get this win. It means a lot. Every single day here is an amazing opportunity. I'm just thinking about having fun and I'm looking forward to my next round."

Aminisova has been rewarded for her win with a fourth-round meeting with world number one Ashleigh Barty, who beat Camila Giorgi in straight sets earlier on Friday.