MELBOURNE, Australia -- Roger Federer got cranky at the chair umpire for a technology flaw in his Australian Open quarterfinal match, using the rare emotional outburst as motivation.
It helped. The 36-year-old Federer, now the oldest semifinalist in Melbourne in 41 years, beat long-time rival Tomas Berdych 7-6 (1), 6-3, 6-4 Wednesday and will next face a challenge from the next generation.
That will be against 21-year-old Hyeon Chung, the first South Korean to reach a Grand Slam semifinal and the youngest to reach the last four at a major since 2010.
Federer's victory extended his winning streak to 14 in Australian Open quarterfinals and to nine in a personal rivalry with Berdych that dates back to 2004. The 19-time major champion leads that head-to-head contest 20-6, including all five meetings at Melbourne Park.
Federer had to overcome a shaky start, dropping his opening service game and uncharacteristically questioning chair umpire Fergus Murphy because of a technological fault. With Berdych serving for the first set in the ninth game, Federer had challenged a line call.
After a lengthy delay, Murphy called the control room and confirmed the replay graphic couldn't be displayed on the stadium screen, and also that the original decision stood. When he added that Federer had no challenges remaining for the set, Federer approached the chair and the crowd cheers intensified.
"Yeah, but you can't steal my challenge," Federer told Murphy. "Do you feel comfortable with this? You're OK with it?"
Seven points later, he eventually broke Berdych to get back on serve, and then won the tiebreaker. The match was as good as over.
"I had to get a bit lucky. A bit angry. A bit frustrated maybe at the umpire," Federer said. "Anyway, glad to get out of that first set. It was key to the match."
Federer later said he just wanted an explanation from the chair, and agreed that blowing off steam helped his cause.
"I was under pressure. It was definitely very close, the turning point," he said. "I played a great breaker. But coming back from 5-2 in the first set, it was clearly big tonight."
Chung beat 97th-ranked Tennys Sandgren 6-4, 7-6 (5), 6-3 in the afternoon match.
He hadn't let up in upset wins over No. 4 Alexander Zverev or six-time Australian Open champion Novak Djokovic, but he let his guard down in the last game against Sandgren and needed six match points to finish it off.
"In last game, I think at 40-love ... if I win one more point, I make history in Korea. I have to think about the ceremony, something," Chung said, explaining how he let his celebrations get slightly ahead of the result. "After deuce, break point. I was like, 'No, nothing to do with ceremony. But just keep playing - keep focused.'"
In an on-court interview, he introduced the audience to his parents and his coach, and took the microphone to speak in Korean to millions of new tennis fans back home.
"I think all the people is watching Australian Open now," he said.
The 58th-ranked Chung is the lowest-ranked man to reach the Australian Open semifinals since Marat Safin in 2004.
With Chung already through, and Kyle Edmund playing No. 6 Marin Cilic in the other half of the draw, it's the first time since 1999 that multiple unseeded players have reached the Australian Open semifinals.
Federer has been keeping an eye on Chung's progress.
"To beat Novak on this court is particularly difficult. ... He's incredibly impressive in his movement, reminds me obviously a lot of Novak," Federer said. "He's clearly got nothing to lose. I will tell myself the same and we'll see what happens."
Chung, who won the inaugural Next Gen ATP Finals title in November, was 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.
The women's semifinals were determined earlier Wednesday, when 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 2016 Australian Open champion Angelique Kerber routed U.S. Open finalist Madison Keys 6-1, 6-2. In the other semifinal match, No. 2 Caroline Wozniacki will play 22-year-old Elise Mertens.
Halep, a two-time French Open finalist, has had a tough 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.
Kerber, the only major champion still in contention, is riding a 14-match winning streak.
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.
"I am just trying to find the feeling back that I had, like 2016," Kerber said, "and just enjoying my time."