Women’s world number one Angelique Kerber was knocked out of the Australian Open on Sunday in a stunning upset by a supremely confident Coco Vandeweghe, as yet another big seed was sent packing early. The German, defending a grand slam title for the first time, had no answers to the big-hitting American who powered past her 6-2, 6-3 on Rod Laver Arena to make the quarter-finals in one of the biggest wins of her career. Kerber’s meek demise follows the early exits of women’s third seed Agnieszka Radwanska, fourth-ranked Simona Halep and sixth-seeded Dominika Cibulkova at a tournament that has been thrown wide open. Unheralded German Mischa Zverev beats world number one Andy Murray to reach Australian Open quarter-finals Top-seeded Kerber was bidding to be the first woman to successfully defend an Australian title since Victoria Azarenka in 2013, but it all went horribly wrong against a player ranked 35. “It’s really special, to play a number one player in the world on any stage and any round. So I think I believe this is my first number one win. I’ll take that,” said Vandeweghe. “Last year I came here and didn’t win a match. Now here I am. It was just another opponent in the way of my goals for this year,” she added. “I set a goal this tournament of making the quarter-finals and the semi-final of a grand slam this year.” Vandeweghe, who made the Wimbledon quarter-finals in 2015 but had never been past the third round in Melbourne, now faces dangerous seventh seed Garbine Muguruza for a place in the last four. Muguruza blasted into her first Melbourne quarter-final with a two-set thumping of Romania’s Sorana Cirstea.