Six-time Australian Open champion Novak Djokovic was beaten by South Korea’s Hyeon Chung, whose superb play had the Serbian struggling for most part of the encounter.
The 21-year-old Chung won 7-6 (7-4) 7-5 7-6 (7-3) to become the first Korean player to reach the last eight of a Grand Slam.
He will face another surprise quarter-finalist after American Tennys Sandgren upset fifth seed Dominic Thiem.
Djokovic, 30, had treatment to his right elbow after the first set and was visibly in pain at times.
The Serb, seeded 14th, missed the second half of the 2017 season with an elbow injury, and was making his return to competitive action in Melbourne.
He made it through three rounds but could not cope with a brilliant performance from Chung, while at the same time clearly struggling physically.
Chung’s ability to turn defence into attack and prevail in epic rallies was reminiscent of Djokovic himself.
“When I’m young, I’m just trying to copy Novak because he’s my idol,” said Chung.