Chung, 21, followed up his fourth-round upset of six-time victor Novak Djokovic by downing Sandgren in straight sets. Berdych had a set point at 5-3; however, Federer saved it and then reclaimed the break. Chung, who squandered a match point on Sandgren's serve in the previous game, jumped up 40-0 on his own serve.
"I thought he played an incredible match against Novak", Federer said about Chung.
Sandgren, who has attracted as much attention for his tennis in recent days, appeared to be struggling but he comeback with an impressive comeback in second set. He sent a text message to the Spaniard before he went to sleep the previous night, adding: "I'm happy the news is not awful".
Chung, who is also the lowest-ranked player in 14 years to get to the last four in Melbourne, speaks limited English but the Australian crowds have taken to him during his candid on-court interviews.
In the women's quarters, Germany's Angelique Kerber continued her march back to top form as the ex-Melbourne champion crushed American Madison Keys 6-1, 6-2.
Defending champion Roger Federer swept into a record 14th Australian Open semi-final with an immaculate straight sets win over long-time rival Tomas Berdych yesterday. But I'm just trying to keep focus on the court and enjoy. Awaiting the victor of this unexpected semifinalist will be Marin Cilic.
With Marin Cilic and Kyle Edmund lining up in the other semi-final, that particular dominance is already destined to be broken and Chung denying Federer a place in his 30th grand slam final remains a huge "if".
"Right now I couldn't tell you how I need to play him".
Novak Djokovic reacts during his fourth-round Australian Open clash against Chung Hyeon yesterday. Chung never previously got past the third round of a major event, while Sandgren began last week with just two career match victories on the ATP Tour, both last summer in Washington.
Federer, the 19-time Grand Slam champion and one of the greatest tennis players of all time, once held the world's top spot continuously for a record 237 weeks, becoming the highest-paid tennis player with earnings of about US$118 billion.
The two later engaged in a long rally that consisted of 31 shots, with Sandgren finding the net to bring the game to deuce. I know that I can win close matches and also, yeah, going out there and playing good in the bigger tournaments.
Elise Mertens is in her first Slam semifinal ever, and on her Melbourne Park debut. Only matches between Federer, Rafael Nadal and Novak Djokovic have been more frequent at the Grand Slams in the Open Era.