He's the soft-spoken South Korean with all the hallmarks of a stereotypical teenager: glasses, braces and a boyish grin.
But on court, Hyeon Chung is a lethal and relentless force with stunning athleticism and a nerve of steel who is wiping the floor with tennis champions.
So just who is the 21-year-old up-and-comer becoming known simply as "The Professor"?
That's the question on everyone's lips after he knocked idol Novak Djokovic out of the Australian Open on Monday night in three stunning sets.
Chung is the first South Korean in history to make the quarter-final of a Grand Slam and is being heralded as one of the greatest players of the next generation.
But the sheepish player who is ranked No.58 insists no one knows who he is - even in South Korea.
"Are you becoming a star in [South] Korea now after winning Milan and today?" one reporter asked after his win against No.4 Alexander Zverev on Friday. "Do they stop you in the street, ask for an autograph?"
"No. Tennis, not that popular in [South] Korea. But in the court, maybe someone want me to take a picture sometimes, yeah. That's the only [time]."
"A lot of girls calling you?" the reporter pressed.
"Do you have a girlfriend?"
"Start," the reporter implored.
"No," said Chung laughing.
But after his win against Djokovic - a champion Chung directly based his playing style on - the South Korean was more confident.
"In my country, I think tennis will be coming up after this tonight," he chuckled.
Chung's name was still trending on Twitter on Tuesday morning as spectators and tennis greats tried to make sense of his incredible win.
"Absolutely stunning performance," wrote tennis great Pat Cash.
"That was an outstanding combination of athleticism and ball striking on a big stage against an all time great," wrote tennis coach Roger Rasheed.
Djokovic was also full of praise after being beaten by the youngster and said he had what it took to make the top 10.
"That was an incredible performance Chung! Keep up great work. You've got this!" he tweeted.
The last time Chung played Djokovic was the first round of the Australian Open in 2016 where Djokovic beat him in straight sets.
Chung's playing style mirrors a predominant Djokovic, who has since been struck down with elbow injuries and is now ranked No. 14.
"That was the most Novak-esque performance I've seen - and it was by Chung!!" wrote Davis Cup champion Jamie Murray. "Truly incredible performance. Wow."
Sports broadcaster Simon Cambers said watching Chung was like watching Djokovic.
"I knew Hyeon Chung was good, but that was unbelievable tennis from a 21-year-old to beat Djokovic. It was like looking in a mirror. Sensational stuff."
After smashing all his peers in the inaugural NextGen ATP Finals in Milan in November, it looks like nothing can get in Chung's way.
He must now take on American Tennys Sandgren who is currently ranked No. 96. It's the first quarter-final either men have played.
Chung took 6 weeks out over wimbledon last year - made some technical adjustments and grip changes. Think they've paid off!??? Jamie Murray (@jamie_murray) January 22, 2018
Amazing performance by Hyeon Chung. So calm, so quick, so good. Two great stories with Sandgren and Chung to play for a spot in the semi's. Looking forward to it already!??? Darren Cahill (@darren_cahill) January 22, 2018
Absolutely standing ovation for brilliant performance from Chung taking out Djoker welcome to the big time??? Brad Gilbert (@bgtennisnation) January 22, 2018