"I definitely didn't expect to win," the 18-year-old qualifier said after a 7-6 (7/3), 6-2 victory over 13th-ranked Croatian Borna Coric advanced him to Friday's semi-finals against US defending champion John Isner.
"I felt like I had margin over him, had a bit of an edge. I just felt really comfortable out there from the first balls."
Away from the ever-brightening spotlight, Auger-Aliassime always had confidence in himself.
"I know I belong here," he said after coming through the Miami qualifiers.
As Coric was dismantled in the quarter-finals of the Masters 1000 event, it was impossible to escape the feeling that it was Auger-Aliassime's breakthrough moment, his run speaking of a coming of age.
Not only did he become the youngest player to reach the last four in the Miami Open's 35-year history, but when the new rankings are released next week, Auger-Aliassime will become the first player born in the 2000s to reach the top 50 of the ATP rankings.
There aren't many miles on his lean frame. Auger-Aliassime looks fresh and hungry and moves with a dynamic game mindful of those by Britain's Andy Murray and Serbian Novak Djokovic, who also went deep in Miami as teens.
Auger-Aliassime's father Sam was born in Togo before emigrating to Canada, where he met Felix's mother Marie, a child psychology teacher.
Sam runs a tennis academy and coached his son until age 13 before the National Training Center in Montreal came calling.
Auger-Aliassime won the 2015 US Open junior title at 15 and knew tennis was the career path for him.
"Since I started walking I was playing because my dad was coaching," he said.
"Tennis has always been part of my life. I don't remember a time when it wasn't. When I started watching Roger Federer and Rafa Nadal on TV, when I was around six years old, I knew this is what I wanted to be."
Auger-Aliassime served notice of his intent by reaching the final in Rio last month and the piano-playing Montreal native isn't hitting any bum notes in Miami either.
"I think I'm seeing the long term," he said. "I'm enjoying every match, because you never know what's going to happen next."
- Canadian teens rising -
His meteoric rise comes at a boom time for Canadian tennis. Bianca Andreescu, also 18, won the title in Indian Wells earlier this month.
And 19-year-old Denis Shapovalov, the world number 23 who has reached a Miami quarter-final, is also being tipped for the brightest of futures.
"Everyone is super excited back home," Auger-Aliassime said. "It's great to hear all these good comments from them. It puts a lot of belief in tennis in Canada.
"I think all the Canadian players from the young kids to Denis and Bianca and I, there's a lot of belief right now. It's great to see."