Hyeon Chung became the first South Korean to reach a Grand Slam quarter-final ©Getty Images

Six-time champion Novak Djokovic suffered defeat in the fourth round of the Australian Open men's singles to an inspired Hyeon Chung in Melbourne.

Djokovic was appearing at his first tournament after pulling out of the 2017 season with injury, and the Serbian had progressed impressively through his early matches.

He found himself immediately on the backfoot against South Korea's Chung, with the 21-year-old breaking twice to take a 4-0 lead in the opening set.

While Djokovic fought back to force a tie-break, Chung ultimately came through to take the opening set to raise the prospect of a shock.

The possibility became increasingly likely when Chung held off a Djokovic fightback in the second to win the set 7-5.

Chung, showing little nerves, impressively held off 14 of the 19 break points earned by the six-time winner during the match.

He was able to complete an impressive victory by winning a third set tie-break, completing a 7-6, 7-5, 7-6 win.

The success saw Chung become the first South Korean player to reach the quarter-finals of a Grand Slam competition.

His reward will be a clash against another unseeded player in the United States' Tennys Sandgren, as the world number 97 upset Austria's Dominic Thiem.

Momentum swung consistently during the match as Thiem twice fought back from a set down to ultimately take the contest into a fifth.

Sandgren eventually emerged as the winner, with the American triumphing 6-2, 4-6, 7-6, 6-7, 6-3.

Tournament second seed Roger Federer had no trouble in reaching the quarter-finals, with the defending champion earning a straight sets win.

The Swiss star overcame Hungary's Marton Fucsovics 6-4, 7-6, 6-2.

Angelique Kerber came from a set down to reach the women's singles quarter-finals ©Getty Images
Angelique Kerber came from a set down to reach the women's singles quarter-finals ©Getty Images

The 19-time Grand Slam winner will now face the Czech Republic's Tomas Berdych, who recorded a 6-1, 6-4, 6-4 win over Italy's Fabio Fognini.

Top seed Simona Halep eased into the last eight of the women's singles draw, with the Romanian wrapping up a straightforward 6-3, 6-2 win over Japan's Naomi Osaka.

She will meet the winner of the match between Czech rivals Barbora Strycova and Karolina Pliskova.

Germany's Angelique Kerber, the 2016 champion, was forced to come from a set down to progress to the last eight.

She ultimately beat Su-Wei Hsieh of Taiwan 4-6, 7-5, 6-2.

Kerber will face American Madison Keys for a place in the semi-finals, with the US player beating Caroline Garcia of France 6-3, 6-2.