AP
South Korea's Chung Hyeon celebrates after defeating United States' Tennys Sandgren in their quarterfinal at the Australian Open tennis championships in Melbourne, Australia, Wednesday, Jan. 24, 2018. (AP Photo/Vincent Thian)

MELBOURNE, Australia (AP) — Serving for a spot in the Australian Open semifinals and with the score at 40-love, Hyeon Chung started thinking how he might celebrate being the first Korean to reach the last four of a Grand Slam.

Not so fast. He hadn't let up when upsetting No. 4 Alexander Zverev or six-time Australian Open champion Novak Djokovic en route to the quarterfinals, but he let his guard down for a few points against No. 97-ranked Tennys Sandgren.

He missed four match points in the last game and had to fend off two break points, including one in a 31-shot rally dominated by slice backhands, before finally beating Sandgren 6-4, 7-6 (5), 6-3 at Rod Laver Arena on Wednesday.

"In last game, I think at 40-love, I'm thinking what I had to do in ceremony or something like that," he said, explaining how he got slightly ahead of himself. "After deuce, break point. I was like, no, nothing to do with ceremony. But just keep playing — keep focused."

Then he fully embraced the moment, joking with Jim Courier in an on-court TV interview, introducing the audience to his parents and his coach, and taking the microphone to speak in Korean to millions of new tennis fans back home.

The No. 58-ranked Chung is the lowest-ranked man to reach the Australian Open semifinals since Marat Safin in 2004. At 21, he's also the youngest to reach the last four at a major since Marin Cilic did it here in 2010.

Two women who've been to this stage at a Grand Slam before will meet in the last four. Top-ranked Simona Halep recovered from an early break to win nine straight games in a 6-3, 6-2 win over No. 6 Karolina Pliskova and set up a semifinal match against 2016 champion Angelique Kerber, who routed U.S. Open finalist Madison Keys 6-1, 6-2.

Kerber has been the only Grand Slam singles champion in the women's draw since her third-round win over Maria Sharapova. Two-time French Open finalist Halep has had a tougher road — having to save match points in a third-round win over Lauren Davis that finished 15-13 in the third — to reach the semifinals at Melbourne Park for the first time.

Not that Chung's run has been routine. After taking out Zverev and Djokovic, Chung could next face defending champion Roger Federer for a spot in the final. Federer was playing Tomas Berdych later Wednesday in the quarterfinals.

Until the last game, Chung had been simply too consistent for Sandgren, a 26-year-old American who had never won a match at a Grand Slam tournament or beaten a top 10 player until last week.

Sandgren's unexpected run — he beat 2014 champion Stan Wawrinka and No. 5 Dominic Thiem en route to the quarterfinals — was overshadowed by heavy scrutiny of his Twitter account and his follows and retweets of far-right activists. He deleted his Twitter history before the quarterfinals, saying he wanted to start from scratch, but must have been distracted by the fallout.

Kerber has had no serious distractions on a 14-match winning streak, and is hoping to emulate her breakout year in 2016.

She won the Australian and U.S. Open titles two years ago and reached the No. 1 ranking, but slipped into the 20s last year. She didn't win a title between the 2016 U.S. Open and the Sydney International earlier this month.

Her first three wins were in straight sets but a fourth-round struggle against No. 88 Hsieh Su-wei had commentators wondering if Kerber was in 2016, or 2017 form.

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She responded with six service breaks against the No. 17-seeded Keys, finishing off the match in 51 minutes and improving her record to seven wins in eight matches against the American.

"I just try to find the feeling back that I had, like 2016, and just enjoying my time. I mean, on and off court," Kerber said of her turnaround. "I know that I was working hard in the offseason, and I know that I can play good matches. I know that I can win close matches and also, yeah, going out there and playing good in the bigger tournaments."