He also happens to be the former coach of her semifinal opponent, Simona Help.
Chung who made headlines for beating former champion Novak Djokovic in the fourth round, ended Sandgren fairy tale run in three sets 6-4 7-6 6-3.
Making a mockery of the 21st seeding she landed after a miserable 2017, the former world number one outplayed Keys in nearly every facet of the 51-minute match to seal a semi-final date with Simona Halep or Karolina Pliskova.
Keys was broken four times in five error-riddled service games as the German raced to the first set 6-1 in 22 minutes and went 3-1 up in the second.
Halep, still looking for her first Grand Slam title, was under the cosh in the opening stages of the match as 72nd-ranked broke the Romanian for a 3-2 lead. I think that's why I could start to dominate the game.
"This gives me a lot of confidence to be here and playing my best".
Sandgren will play the victor of the fourth-round match between six-time champion Novak Djokovic and being played on.
Hsieh saved two match points on her serve in the seventh game of the third set, forcing Kerber to serve out the match in the next game.
"Madison is a hard hitter, and she served good".
The pair have, however, played on six previous occasions with Halep leading the ledger 5-1.
"I have to be strong in my legs and be calm and start playing like I did at the end of the match".
"I'm going to have to be the aggressive one and take time away, but also be really smart", Keys said. Then I started to open the court and play my style.
"I have the plan in my head".
"The feeling in my muscles is gone and I don't feel my ankle any more", she added. "I'm enjoying being on court", she said.
Simona Halep displayed tennis true to her world No.1 tag at the Rod Laver Arena on Wednesday as she romped past sixth seed Karolina Pliskova to reach her maiden Australian Open semi-final.
On the men's side, an injured and visibly struggling Rafael Nadal retired while trailing in the fifth set of his quarterfinal match against Marin Cilic. "The MRI (scan) showed a grade one injury of his illiopsoas on his right leg", his management said in a statement.
"Once again tennis fans will be able to enjoy many great matches, including some of those players who have thrilled so many with their performances in Australia this month", said Tournament Director Salah Tahlak. Keys says the sadness lasted a while, for the obvious reason: When you come oh-so-close to winning your first Grand Slam title and crumble, nothing feels worse in tennis.
"It would be not for sure, but most probably my set, the first one".