MELBOURNE: Top seed and reigning champion Angelique Kerber was bundled out of the fourth round of the Australian Open by big-hitting American Coco Vandeweghe on Sunday, losing 6-2 6-3 on Rod Laver Arena.

The German world number one's exit followed that of men's top seed and world number one Andy Murray, who lost to Mischa Zverev earlier in the evening on the same court.

The 25-year-old New Yorker claimed a place in the quarter-finals at Melbourne Park for the first time, blasting six aces and 30 winners to overpower Kerber.

"I guess I faked (having confidence) a lot because I was feeling like crap out there," said the American.

"But, you know, fake it 'til you make it' .... my game plan was to execute knowing that she was going to get a lot of balls back."

The world number 35 smashed a backhand crosscourt winner to take the single break of serve she needed to claim the opening set in 33 minutes.

Kerber, who also won the US Open last year, hit back by breaking her American opponent on a double fault in the opening game of the second set but Vandeweghe would not be denied.

Taking advantage of a series of uncharacteristic Kerber errors, Vandeweghe ramped up her big forehand to break back for 3-3 and another backhand gave her the opportunity to serve for the match.

Kerber was by now looking broken and sealed her fate, and Vandeweghe's victory, when she looped a return far beyond the baseline.

Vandeweghe, who celebrated her biggest career victory with a shrug of the shoulders, moves on to meet Spanish seventh seed Garbine Muguruza in the last eight.

"Last year I came here and didn't even win a match and here I am now," Vandeweghe added.

"It's confidence having a full off-season with no injuries .... I set a goal for this tournament to make the quarter-finals."