Hyeon Chung shocked six-time champion Novak Djokovic with a straight-sets victory to set up an unlikely Australian Open quarter-final against Tennys Sandgren.
Former world No 1 Djokovic, who has been playing with a sleeve over the troublesome elbow that sidelined him for six months, paid the price for a lack of competitive tennis as he suffered a 7-6 (7-4) 7-5 7-6 (7-3) defeat to relentless 21-year-old Chung.
Chung, who won the inaugural Next Gen ATP Finals in November, lost in three sets to Djokovic at the Australian Open in 2016, but he ended the Serb's hopes of reaching the last eight in Melbourne for a 10th time.
"I didn't know how I'd win tonight. I was just honoured to play against Novak and happy to see him back on the Tour," said Chung, who became the first South Korean ever to reach a Grand Slam quarter-final. "I try to copy Novak in the way I play, he is my idol."
Sandgren, who is ranked 97 in the world, only made his Grand Slam debut at the French Open last summer and had never won a Slam match before arriving in Melbourne.
The 26-year-old American, who is named after his Swedish great-grandfather, broke the Thiem serve in the sixth game of the deciding set to win 6-2 4-6 7-6 (7-4) 6-7 (7-9) 6-3.
"I'm starting to disbelieve it. I don't know if this is a dream or not. But all you guys are here and I'm not in my underwear so maybe it's not," said the Tennessee-born player.
"He played some really great tennis, especially in the fourth-set breaker. I knew I had to take my chances. I knew he probably could out-grind me, he's a heck of a player. Thankfully it worked out in the end."
Sandgren is only the second man in 20 years to reach the quarter-finals on his debut after Alexandr Dolgopolov in 2011.
"I have that going for me that guys don't know how I'm going to play and I'm using that to my advantage just to keep riding that wave," he said. "I've lost in qualifying four years in a row and now I'm in the quarter-finals."