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South Korea's Hyeon Chung celebrates after a point against Serbia's Novak Djokovic during their men's singles fourth round match on day eight of the Australian Open tennis tournament in Melbourne. Image Credit: AFP

Melbourne: Wounded 12-time Grand Slam winner Novak Djokovic was sent tumbling out of the Australian Open by Korean giant-killer Chung Hyeon on Monday, but ice-cool Roger Federer roared into the quarter-finals.

The Serb, who was returning from six months out with a right elbow injury, was clearly in pain as his hopes of a seventh Melbourne Park title were extinguished on Rod Laver Arena 7-6 (7/4), 7-5, 7-6 (7/3).

He was keen not to take away from the glasses-wearing Chung’s huge achievement after the 21-year-old followed up his win over fourth seed Alexander Zverev in the last round.

“Congratulations. Amazing,” said Djokovic, who also had an apparent hip or groin problem, screeching at times when stretching for the ball.

“He was the better player on court today and he deserved to win.”

Djokovic said the injuries started troubling him at the end of the first set and a decision on where he goes from here would be made after talking with his medical team.

Chung’s reward is a last-eight clash with unheralded American Tennys Sandgren, who upset Austrian fifth seed Dominic Thiem 6-2, 4-6, 7-6 (7/4), 6-7 (7/9).

Sandgren becomes just the second man in the last 20 years since Frenchman Nicolas Escude to make the quarter-finals on his Australian Open debut.

Amazingly, the 26-year-old missed out on qualifying in the last four years to reach the main draw in Melbourne.

Sandgren beat former winner Stan Wawrinka in the second round and this time eliminated the world No.5.

“I don’t know if this is a dream or not,” Sandgren said on court. “He played some really great tennis, especially in that fourth set tiebreaker.

Chung, ranked 58, described Djokovic as “my idol”.

“I can’t believe this, dream’s come true tonight.”

Sandgren was also in a daze, becoming just the second man in the last 20 years since Frenchman Nicolas Escude to make the quarter-finals on his Australian Open debut.

“I don’t know if this is a dream or not,” he said. “I knew I had to come out and play aggressive and take my chances and serve well and thankfully it worked out in the end.”

For a composed Federer, it was business as normal. The Swiss defending champion swatted aside one-time training partner Marton Fucsovics 6-4, 7-6 (7/3), 6-2 to make the last eight for an incredible 14th time.

He will now play fellow veteran Tomas Berdych, a semi-finalist in 2014 and 2015, who demolished Italy’s Fabio Fognini 6-1, 6-4, 6-4.

“We have had some good ones over the years going back all the way to the Olympic Games in Athens in 2004,” the 19-time Grand Slam champion said of the Czech.

“Yeah, I’m looking forward to play against him. He seems in good shape.”

Berdych, who is on his way back from back problems, lost to the five-time Australian Open champion in the third round last year and the odds are heavily stacked again him.

Of the 13 quarter-finals Federer has contested in Melbourne, he has won the lot.