It was a win that came sooner than expected.
After holding in the first game of the third set, Popyrin brought up break point with a return winner, and converted that when Thiem dumped a tired-looking backhand into the net.
The Austrian – who had been stretched to five sets late on Tuesday night against Benoit Paire in the opening round – shook his head, and conceded defeat.
"I felt very bad during the game, obviously," Thiem said.
"Yesterday after the long match I felt really fine, actually. (But) started to feel like my whole body hurt, general not feeling well at all, and in the match it got worse. Whole body was hurting, I was feeling weak, and there was no sense to continue."
Thiem began showing signs of physical distress early in the second set when he asked for the trainer, telling the umpire he was experiencing fatigue. Medical staff came arrived at the next changeover to offer him paracetamol for soreness and jelly beans for energy.
It’s not clear whether he took them or not, but he seemed to loosen up as the set progressed and began to trouble the young Aussie with the sting in his groundstrokes.
Games were now humming along on serve, with Thiem cruising to a 40-0 lead in the 10th game. Yet Popyrin unleashed his own powerful winners to level at deuce.
Two errors from Thiem followed, and suddenly the teenager had snared the break – and a two-sets-to-love lead.
Whatever the extent of Thiem’s physical woes, there was no denying the intent with which Popyrin arrived at Melbourne Arena. “I live for the big stage … and just (want) to play against a great player on that big stage, really,” he had told Channel 9’s Wide World of Sports before the match.
It is usually top-10 star Thiem – a Roland Garros finalist last year – winning lengthy, intense slugfests from the back of the court. Yet Popyrin held firm in a taxing ninth game extending seven deuces, surviving four break points in the process.
It sent a resounding message down the other end of the court to Thiem, who dropped serve to love three games later to surrender the opening set.