MELBOURNE (Reuters) - South Korean Chung Hyeon does not appear especially intimidating behind his Zen-like demeanor and white-framed spectacles but as German wunderkind Alex Zverev will testify, the 21-year-old should come with a health warning.

Tennis - Australian Open - Rod Laver Arena, Melbourne, Australia, January 20, 2018. Alexander Zverev of Germany waves as he leaves after losing against Chung Hyeon of South Korea. REUTERS/Thomas Peter

Fourth seed Zverev went into their Australian Open third-round clash of rising stars as favorite and with many tipping him as a potential winner of the tournament.

Despite winning the first and third sets with some stunning tennis, however, the 20-year-old could not shake off the relentless Chung and ended up caving in 5-7 7-6(3) 2-6 6-3 6-0.

Zverev was a broken man by the time the deciding set began and Chung needed just 23 minutes to put him out of his misery to become just the third South Korean to progress to the fourth round of a grand slam.

A match that had started with a razor-sharp Zverev playing exemplary tennis ended with him complaining about the light and smashing rackets in rage.

It was hard to blame him though, as playing 58th-ranked Chung must have felt like slamming your head into a brick wall -- such is his court coverage and ability to soak up punishment.

Zverev, for all the hype and victories over Novak Djokovic and Roger Federer in Masters Series finals last year, has still not gone beyond the fourth round of a grand slam.

“I have some figuring out to do, what happens to me in deciding moments in grand slams,” he told reporters. “It happened at Wimbledon, it happened in New York. It happened here. But I’m still young, so I’ve got time.”

The level of tennis produced in the first three sets was so high it looked like a future grand slam final.

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LIGHTS OUT

“He’s 50-whatever in the world. But this was a top-10 level match from the start till the end of the fourth set, and for him until the end,” Zverev said.

“When he plays like that, there are very, very few people who will beat him.”

Six-times champion Djokovic, Chung’s next opponent, will no doubt heed the warning.

There was nothing between the players in the first set until Zverev piled on the pressure in the 12th game with a couple of rasping baseline winners before prevailing in a 26-stroke rally that ended with a Chung error.

No quarter was give in the second set but Chung, who beat Zverev’s brother Mischa in the first round after he retired ill, leveled with five successive winning points in the tiebreak.

Zverev found another gear to break twice in the third but the unflappable Chung was far from finished, and broke early in the third set, before consolidated it to lead 3-0.

The first sign that Zverev was beginning to unravel came when he berated the umpire for the fact that the lights were not switched on, despite it being only around 6 p.m. -- at least two hours before dusk in Melbourne.

“I can’t see anything,” Zverev barked. “All you have to do is flick the switch.”

The lights did eventually come on but they were soon turned off for Zverev as he double-faulted to gift Chung a break in the first game of the deciding set.

When Zverev looped a forehand into the net two games later to concede a second break, his racket paid the price, ending up in a crumpled heap next to his chair.

Chung barely changed his expression, quickly sealing his first victory over a top-five opponent to advance.

“I’m happy to share the court with Novak,” he said bashfully of his next challenge. Looks can be deceiving, though.