The point of the match came in the third-set tiebreaker when Chung hit a cross-court passing shot to put him within two points of victory. I have to serve well, otherwise she has, I would not say risky game but if I don't play well I don't have the chance to win against her.
Six-time champion Novak Djokovic was stunned in straight sets by Hyeon Chung not long after Tennys Sandgren upset No. 5 Dominic Thiem at the Australian Open.
The Romanian reeled off 9 games in a row and 12 of the last 14 games to recover from 0-3 en route to a 6-3, 6-2 victory in 1 hour and 11 minutes on Rod Laver Arena.
In the semi-final on Friday, Halep will be up against former world No.1 Angelique Kerber, who earlier thrashed Madison Keys 6-1, 6-2.
The World No.6 has had an easy run through the first grand slam of the year, dropping just a single set.
What Facebook's Feed Changes Mean for the News
Last year Facebook came under considerable scrutiny after being accused of being the engine of fake news during the US elections. The social network has had trouble managing its role as one of the world's most powerful news distributors.
The 22-year-old threatened to mount a recovery when she pulled back from 3-0 to 3-2 in the second but, after her serve was broken again in the next game, the match quickly went away from her.
She took nearly three hours to get past fellow Czech Barbora Strycova in a last-16 match that did not finish until just before 2:00 am on Tuesday.
This is set to be an intriguing match, but with possibly fitness issues and the amount of time Halep has already spent on court, the world No.1 will be sent packing. "I served well so everything went pretty well for me".
Halep, 26, made few mistakes to set up a last-four clash against Angelique Kerber. "I just want to play my best". Maybe I get used to the pain and I'm not thinking that much that something can happen.
"To have such a great two weeks and then have it end the way that it did, it was really devastating for me, so it definitely took some time to get over", she said of her US Open runner-up effort.
A double fault and 2 forehand errors cost Karolina another service game while serving at 2-5 and that break pushed Simona over the finish line, cruising through the semis and looking to maintain this level against a former victor Angelique Kerber as well.