Popyrin let out a huge roar after sealing his victory in front of 250 fans, who were allowed to attend the tournament after passing COVID-19 tests.
"I worked my whole life to get into an ATP final and I'm finally in one tomorrow. It's definitely an amazing feeling but I got one more match to go and I want to focus on that," world No.114 Popyrin said.
After taking the second set to a tie-break, Popyrin raced to a 5-0 lead with his huge serves, helped no less by two unforced errors from Cilic.
The former world No.3 managed to stage a fightback to 2-5 but another ace from Popyrin swung the advantage back to the Australian, giving him four match points.
A powerful serve followed up by a forehand winner eventually saw Popyrin winning in just less than two hours.
"Only 250 fans are allowed but they were loud and it felt like the stadium was full. There were some Aussies I heard from the accent and they were screaming, 'Come on Popy'. That's always an amazing feeling when you're not playing in your home country," Popyrin said.
Kazakhstan's Alexander Bublik sent Moldova's Radu Albot packing in the day's other semi-final match with a 2-6 6-3 6-4 victory to head into his fourth ATP final, where he will take on Popyrin.