Hyeon Chung is the first Korean to reach a grand slam semifinal after continuing his spectacular Australian Open run at Melbourne Park.

The 21-year-old world No.58 outclassed unseeded American Tennys Sandgren 6-4 7-6 (7-5) 6-3 on Wednesday to also become the lowest-ranked man to make the last four of the Open since Marat Safin in 2004.

Chung will play second-seeded titleholder and five-time champion Roger Federer or Czech Tomas Berdych on Friday night for a most improbable berth in the final.

"Whoever win, I'm playing. I don't care," a jubilant Chung said.

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Backing up from his stylish straight-sets victory over six-time winner Novak Djokovic, Chung showed no signs of a psychological letdown as he broke Sandgren four times on Rod Laver Arena to storm into the last four in two hours and 28 minutes.

Triumphant at the 2017 Next-Gen Finals in Milan, Chung has carried his blazing form to Melbourne, adding Sandgren to his growing list of scalps this fortnight that also include world No.4 Alexander Zverev, the German's brother Mischa who ousted Andy Murray last year and Sydney International champion Daniil Medvedev.

At 21 and 254 days, Chung is also the youngest men's grand slam semifinalist since Marin Cilic at the 2010 Australian Open. The South Korean could meet the Croat who in this year's final.

While Chung marches on, Sandgren can console himself with a cheque for $440,000 and a surge from 97th in the rankings to the world's top 60.

The unfancied American had never won a match at the majors, failing on 13 previous occasions to qualify for a grand slam.

The 26-year-old fought off five match points. He finally relented after a thrilling nine-point game that featured one epic 31-point rally and then another extraordinary display at the net from Sandgren in a desperate attempt to stave off defeat.

"I don't know in last game 40-love up, I start thinking what I had to do in ceremony, something like that," Chung said.

"After the deuce point, no ceremony. I'm just trying to stay focused because I'm first time in these (big) matches."

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