Bespectacled like Clark Kent but playing like Superman at a sunbathed Rod Laver Arena, world number 58 Chung became the lowest ranked player into the last four at Melbourne Park since Marat Safin in 2004.
The muscular 21-year-old will meet either champion Roger Federer or Tomas Berdych for a place in the final, and should head into that clash brimful of confidence.
As in his fourth round elimination of six-times champion Novak Djokovic, Chung had all the answers against world number 97 Sandgren, returning virtually everything the battling American could throw at him.
A near-perfect display finished with a wobble when serving out the match, as he squandered a 40-0 lead and saw a total of five match points evaporate into a cloudless sky.
He later admitted to thinking prematurely about victory and what it might mean for his expectant nation.
"I think (in the) last game, many things come together. If I win one more point, I make history in Korea," he told reporters.
"Something I (was) thinking like that. I have to think about the ceremony. Anyway, I had to stay calm because... the match was not finished yet."
Chung's raw power ultimately prevailed, a blazing forehand into the corner causing a scrambling Sandgren to push his desperate retrieve well past the baseline.
Chung flashed a broad grin as South Koreans draped in flags roared in the terraces and his mother shaped a love heart with her arms from the players' box.
South Korea has been late to the party in global tennis but Chung's success could light a fire for the sport in the east Asian nation.
It has also been a devastating raid by a player barely spoken about before the tournament, with most of the buzz around young guns like Germany's Alexander Zverev, who Chung dismantled in the third round, and Australian Nick Kyrgios.
FALSE ALARM
The match began in an unnerving way for all concerned, a false alarm ringing out and warning to "evacuate, evacuate, evacuate" early in the first set.
Play was halted for a few moments and some fans scurried for the exits.
Chung broke Sandgren in the third game and served out the set to love, then took the heavy-legged American's serve again to take a 2-0 lead in the second.
Sandgren rallied strongly, breaking Chung twice but he faltered when serving for the set at 5-3.
The Korean converted a second break point to get back on serve and the pair fought furiously in a breathless tiebreak.
Sandgren wound his forehand up for a straightforward winner to secure two set points but instead thumped the ball into the net to allow Chung to level at 5-5.
Another rush of blood saw the American fire long, and a clinical Chung needed only one chance to take the set.
The Korean flapped his hands at the crowd in triumph, as a red-faced Sandgren retired to his chair to shake his head and fume.
The American appeared spent and slumped to a 3-1 deficit, but gave the crowd their money's worth with brilliant volleying to save a fifth match point.
The 26-year-old bowed out to warm applause from the crowd, though a storm over his social media links with right wing nationalists raged on in the background.
"I'm happy with being resilient," Sandgren said.
"It's not easy to come off some big wins, biggest wins of my career, crazy stages, like quarter-finals of a slam... and to deal with the stuff off the court as well."
(Editing by Greg Stutchbury/John O'Brien)
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