The unseeded made his main draw debut in all the four majors last year and lost in the first round but has been a revelation at Melbourne Park, beating 11th seed Cameron Norrie in five sets and downing Auger-Aliassime 4-6 6-3 7-6(2) 7-6(3).
"I didn't expect that," world number 71 Lehecka, who made his breakthrough last year by reaching the Rotterdam semi-finals as a qualifier, told reporters.
"But deep inside, me and my team, we believe I have the game and that I'm able to produce good tennis, which can take me to higher places...to better rankings, fuller in the tournament.
"Here we are. We're in the quarters. Hopefully we'll go one more step, step after step. We'll see how it'll go... We'll try to prepare as much as we can for what's coming next."
Lehecka's father Jiri was a professional swimmer and his mother Romana a professional track-and-field athlete, which explains his natural athleticism on the court.
After an extended spell on the ATP Challenger circuit, the youngster broke into the top 100 last February and also reached the title clash at the Next Gen ATP Finals in November where he lost to American Brandon Nakashima.
"I think at the Next Gen I finally found my game and I found how to play what I want to play, and how to beat great guys in a big stadium," Lehecka said.
"I saw all the media attention over there. It was some kind of a gate to the big tennis. They showed us over there what it looks like when you're a pro player."
Auger-Aliassime was particularly impressed by his opponent's composure during their meeting on Margaret Court Arena.
"To be in those moments and be cool under pressure, play well the right points, serve well to close the match, it's a good sign of a good player," Auger-Aliassime said.
"Yeah, I'm sure for those reasons he'll be a good player in the future."
Up next for Lehecka is a potential meeting with third seed Stefanos Tsitsipas, his conqueror in the Rotterdam semi-finals, if the Greek gets past Jannik Sinner.
"I'll be super excited. I'll go for revenge, for sure. I know he'll remember how we played last year in Rotterdam. One set I was the better player on court. Then he overtook the match," Lehecka said.
"But he'll know what my strengths are. He'll feel I can get him under the pressure."
