South Korean star Chung Hyeon dumped six-time champion Novak Djokovic out of the Australian Open in a sensational fourth round upset on Monday. The 21-year-old beat the battle-weary Serb, who was clearly in pain as he stretched for balls 7-6, 7-5, 7-6 in a night match on Rod Lever Arena.Chung, the first South Korean man to reach the round of 16 at the Australian Open, will face American bolter Tennys Sandgren in the quarter-finals on Wednesday.Meanwhile, Roger Federer stormed into quarter-final with Tomas Berdych as former champion Angelique Kerber stuttered into an enticing clash with Madison Keys.The defending champion Federer defeated Marton Fucsovics 6-4, 7-6, 6-2 to make the last eight for an incredible 14th time.The Swiss maestro, has made at least the last eight every year since 2004, barring a hiccup in 2015. Second-seed Federer will now play fellow veteran Berdych, a semi-finalist in 2014 and 2015 at his most successful major, who demolished Italy's Fabio Fognini 6-1, 6-4, 6-4.Germany's Kerber is the only Grand Slam champion left among the women and she was given a big fright as she chases her second Melbourne Park crown after beating Serena Williams in 2016. After crushing Maria Sharapova in the last round, she came up against gritty Taiwanese veteran Hsieh Su-wei who stunned the 21st seed by taking the first set on Rod Lever Arena.