Chung, 21, beat Djokovic 7-6 (4), 7-5, 7-6 (3) to reach the Australian Open quarterfinals. He faces American Tennys Sandgren in the next round. Sandgren is ranked No. 97 in the ATP rankings. Djokovic was ranked No. 14.
Chung holds the No. 58 spot in the ATP rankings. He turned pro in 2014 and owns a 54-45 record. He is 3-2 this season, with $22,545 in prize money.
His lone title victory came in November at the Next Gen ATP Finals in Milan, Italy.
Djokovic is a 12-time Grand Slam champion. He beat Chung in straight sets during the first round of the 2016 Australian Open.
"I didn't know how I'd win this tonight. I was just honored to play against Novak, and happy to see him back on the tour," Chung said after the match in his interview on the court.
"When I was young I just tried to copy Novak because he was my idol."
"I can't believe this. Dreams come true tonight."
"First of all, I have to say that I'm very grateful that I had a chance to play," Djokovic said after the match. "So I played four matches here. It was a good tournament of course. It's disappointing to go out in the fourth round, but the circumstances are such and I have to accept it."
Djokovic was playing in his first tournament on the tour since taking a six-month layoff due to an elbow injury.
Sandgren and Chung face off on Wednesday.