Roared on in Melbourne by Brazilian fans decked out in yellow, the prodigious 18-year-old appeared totally unfazed as he stunned the Russian 7-6 (1), 6-3, 7-6(5).
"It's the first time playing in a huge stadium," Fonseca told an energised Margaret Court Arena after his win.
"There are a lot of Brazilians here cheering for me and I enjoyed every moment, thanks a lot."
Fonseca last month became the second-youngest champion of the NextGen ATP tournament since current world number one Jannik Sinner claimed the title, also aged 18, five years ago.
Fonseca won a second ATP Challenger Tour title in Canberra this month, then powered through qualifying at Melbourne Park to take his place in his maiden Grand Slam main draw.
Ranked 112 in the world, Fonseca came into the first-round meeting with Rublev on a 13-match winning streak.
He had never faced a top-10 player before, but if he was nervous he did not show it as they went toe-to-toe before Fonseca clinched the first set emphatically on the tiebreak.
The Brazilian raced into a 3-0 lead in the second, at one point beckoning the Margaret Court Arena crowd to show some more appreciation.
Fonseca took the second set with an ace.
He was in trouble in the third when Rublev broke for a 3-1 lead, but the youngster broke back immediately to put his more experienced opponent back under pressure.
They went to the tie break again and Fonseca raced into a 4-0 lead, then staved off a mini Rublev fightback to seal the deal on the first match point.
In the lead-up to the opening major of the year, the sometimes combustible Rublev opened up on his battle with anxiety and depression.
During his career, he has reached the last eight at all four Grand Slams but has never gone further.