Defending champion Angelique Kerber bowed out in the Australian Open fourth round after being blown away by American Coco Vandeweghe.

Kerber was the second number one seed to crash out on a day of shocks in Melbourne after Andy Murray had earlier been beaten in the men's tournament by German Mischa Zverev.

Vandeweghe was a worthy winner too on Rod Laver Arena, winning 6-2 6-3 in just 68 minutes to book a quarter-final showdown with Garbine Muguruza.

Ranked 35th in the world, the American lost in the first round here 12 months ago, just as Kerber was beginning her path to become champion.

The world number one, however, has endured a difficult start to 2017 and her fragile confidence was ruthlessly exposed here by a powerful display of baseline hitting.

"It was really special," Vandeweghe said. "Beating the world number one on any stage and any place is great... I'll take this one.

"I wasn't feeling confident, I guess I faked it. I was nervous, but I had a game plan to execute and I knew that as long as I keep picking my spots, I had a chance."

It is the first time Vandeweghe has made the quarter-finals at Melbourne Park and at the finish the 25-year-old celebrated by holding her arms outstretched and shrugging her shoulders.

"In terms of career wins, this is high," Vandeweghe said.

"It's the first time I've ever beaten a world number one and especially at a grand slam, I think it makes it that much sweeter. So it's pretty high definitely."

Kerber's defeat continues her poor start to the season after she lost her first match at Sydney and second in Brisbane earlier this month.

The German will also now surrender the world number one spot she claimed in September to Serena Williams, if the American goes on to win the Australian Open.

"I made a lot of mistakes. I think that was not my game I play normally," Kerber said.

"It was a tough match and of course I'm disappointed. But I was not feeling the ball at all tonight. I was not playing good from the first point.