190120 Danielle Collins
Danielle Collins of the US celebrates during the match against Germany's Angelique Kerber. Image Credit: Reuters

World number two Angelique Kerber was dumped from the Australian Open by unseeded American Danielle Collins Sunday in the biggest upset yet at this year's season-opening Grand Slam.

Collins humiliated the German three-time major champion 6-0, 6-2 in less than hour to reach the quarter-finals on her debut appearance in the Melbourne Park main draw.

The 25-year-old, who will meet either fifth seeded compatriot Sloane Stephens or Russia's Anastasia Pavlyuchenkova in the last eight, said she decided to "go after it and believe in my shots".

"I go out fearless, I've been working so hard my whole life, I just give it my all and don't really think twice about it," she said.

Ranked 35 in the world, she is little known on the WTA Tour after playing much of her tennis in the US college system.

Before this year's tournament she had never won a match at a Grand Slam in five attempts and Kerber trounced her 6-1, 6-1 in their only previous meeting last year.

The delighted American, who also eliminated two seeds in the opening rounds, said she was developing a taste for success at the majors.

"I may not have won a Grand Slam match before this, but I gotta tell you, I think it's going to keep happening," she said. "I hope to have many more of these."

Wimbledon champion Kerber entered the match as overwhelming favourite after not dropping a set in her opening three matches.

She won the title at Melbourne Park in 2016 and many were tipping her to succeed again after she reached the semi-finals last year.

But the German, who turned 31 on Friday, found herself a set down against Collins after just 20 minutes as the American fed her illustrious opponent a dreaded 6-0 "bagel".

Kerber committed five double faults and failed to win a single point on her second serve in the set, while Collins smashed 13 winners to two.

Her agony continued in the second set, when Collins made three breaks to one for the biggest win of her career.