Popyrin has the break in the fourth set after some epic winners from both men.
He won it with a commanding forehand winner screaming before it had even passed Djokovic - he knew he had hit it perfectly.
Early on, Popyrin sprinted to make a baseline shot then back into the net to hit a cross-court winner.
Alexei Popyrin, of Australia, returns a shot to Novak Djokovic, of Serbia.Credit: AP
It gave him a break point against Djokovic but the No.2 seed came up with a winner to save it, turning his ear to the crowd to hear the roar.
Popyrin answers back with a stunning, reaching lob from the baseline which Djokovic left, only to watch it land in.
Djokovic responded to that one with an ace and looked on course to hold but he couldn’t finish the job.
Popyrin is serving at 6-4, 6-4, 2-6, 3-2.