The 20-year-old De Minaur is the youngest finalist in the tournament history.
Although the Aussie endured a string of early exits on clay and grass while dealing with a groin injury, he's 14-4 on hard courts this season and has looked confident on his best surface in Atlanta, reports Xinhua news agency.
"I'm really proud of where I am after suffering a tough injury. I'm back where I want to be," said De Minaur on Saturday. "I felt like I've been playing some great tennis. I knew that sooner or later, I'd have a good result again."
There were no service breaks in the first two sets in the semifinal. De Minaur raced through the last three points of the first set tiebreak, but Opelka hung tough and won a wild second set tiebreak.
After more than two hours of play, De Minaur earned the first break of the match to lead 3-2 in the deciding set. He broke once more to close out the match after two hours and 29 minutes.
Awaiting him in the final is second-seeded American Taylor Fritz, who overcame Cameron Norrie of Britain 6-1, 3-6, 6-3 in the other semifinal.
Fritz said: "I know I'm improving, so it feels really good. I'm playing way better from the last time I played against De Minaur, so It's going to be an exciting match. I'm sure we'll be seeing a lot more of each other in the future as well."
( With inputs from IANS )