Defending champion Osaka knocked out of Australian Open

Defending champion Naomi Osaka has been stunned by unseeded Amanda Anisimova in the Australian Open, the American setting up a fourth-round clash with world number one Ashleigh Barty.

The Japanese four-time Grand Slam champion, seeded 13, won the first set on Margaret Court Arena but the nerveless Anisimova saved two match points in the third to pull off a huge 4-6, 6-3, 7-6 (10/5) upset on Friday.

"I'm speechless, I can't stop smiling," said the 20-year-old, who is undefeated this year after winning a lead-up tournament in Melbourne.

"I knew I had to be playing sharp if I wanted to give myself a chance. Naomi is always going to be playing well, she is an absolute champion.

"So I really had to step up my game and try to be aggressive. That's what I started doing in the second set and so grateful I was able to do this well today and get the win. It means a lot."

Anisimova’s reward is a shot at a place in the quarter-finals against Barty, who crushed Italian 30th seed Camila Giorgi in straight sets.

READ MORE: Osaka, Barty likely to clash in fourth round as Nadal rolls on

Match ended with an ace

Osaka wasted little time getting on top of world number 60 Anisimova, breaking her immediately and then worked two break points on the American's second serve, but failed to convert.

But Anisimova's nerves settled and she began to find her range, threatening with some sizzling groundstrokes, only for Osaka's power and precision to come to the fore.

Despite only getting 59 percent of her first serves in, Osaka was on top and sealed the set with a half-volley.

She faced three break points in a crunch second game in set two, but kept her cool to repel them only to be broken in game four as her serve again let her down.

A pumped up Anisimova took the set, hitting twice as many winners as Osaka.

Both players had chances in a tense third set with neither giving an inch until Osaka worked two match points at 5-4.

But Anisimova saved them both then slammed an ace to level it up at 5-5 and it went to a tie-break where she held her nerve to snap a nine-match losing streak against top 20 players.

READ MORE: Osaka, Barty likely to clash in fourth round as Nadal rolls on