Chung, 21, followed up his fourth-round upset of six-time winner Novak Djokovic by flogging Sandgren in straight sets.
Sandgren drew fire on Tuesday after quickly deleting years of a Twitter feed allegedly featuring racist and homophobic tweets as well as exchanges with alt-right figures.
The world No. 97 tried to defend himself by insisting that his feed did not necessarily reflect all of his views. But later the same day he went on a deleting spree.
The devout Christian refused to take questions on the issue at a post-match press conference, instead reading from a statement.
"With a handful of follows and some likes on Twitter, my fate has been sealed in your minds. To write an edgy story, to create sensationalist coverage, there are a few lengths you wouldn't go to to mark me as the man you desperately want me to be," he said.
Looking at the match, he added: "I'm happy with being resilient. It's not easy to come off some big wins, biggest wins of my career, crazy stages, like quarterfinals of a slam, that's crazy to me, and to deal with the stuff off the court as well."
Chung kept it strictly business on court as he demolished Sandgren in just under two and a half hours, winning on his sixth match point as Sandgren returned long.
"In the last game many things came together," Chung said. "If I win one more point, I make history in Korea.
"I was thinking like that. Anyway, I have to stay calm because the match is not finished yet, so I'm just trying to stay calm until I finish the match."
Kerber, who won the event in 2016 and later claimed the top WTA ranking, continues to erase bitter memories of last season when she fell into a slump and exited the top 20.
The 30-year-old has yet to lose this year, after taking Germany to the final of the Hopman Cup before lifting the Sydney WTA title.
Her defeat of US Open finalist Keys, who has barely played a match due to injury since her September success, was her sixth win from seven meetings between the two.
She leads the series against Halep 4-3, with the Romanian recovering from 3-0 down in the first set with nine unanswered games in her victory over Pliskova.
"It was not my best start," said Halep. "I knew I had to re-start. I stopped missing and moved better. I did pretty well in the end."
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