Fearless 18-year-old qualifier Joao Fonseca declared he wants "more and more" after beating ninth seed Andrey Rublev in straight sets on his Grand Slam debut on Tuesday in a major Australian Open shock.

Roared on in Melbourne by Brazilian fans decked out in yellow, the prodigious teenager appeared totally unfazed as he stunned the Russian 7-6 (1), 6-3, 7-6 (5).

Fonseca last month became the second-youngest champion of the NextGen ATP tournament since current world No 1 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.

"I think when I arrived here my first goal was to qualify for the main draw," said Fonseca, who is ranked 112 in the world and came into the first-round meeting with 27-year-old Rublev on a 13-match winning streak.

"Of course, my expectations are bigger now. I want more and more.

"I think that's the mentality of the champion," added Fonseca, who faces Italy's Lorenzo Sonego next.

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.

Unleashing some thunderous groundstrokes, the Brazilian raced into a 3-0 lead in the second set, 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 tiebreak again and Fonseca powered into a 4-0 lead, then staved off a mini Rublev fightback to seal the deal on first match point.

"My first win in a Grand Slam, my first win against a top-10 player, my first time playing in the main draw of a Grand Slam," said Fonseca.

"So everything's new for me, but I played such a good match.

"I'm just very happy with myself, very proud the way that I fought and the way that I stayed mentally on every game, every 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 never gone further.

Hitting serves at up to 133 mph (214 kph), Fonseca delivered 14 aces and had just one double-fault while saving five of the six break points he faced.

He accumulated 51 total winners, 18 more than Rublev, a quarterfinalist in Melbourne three of the past four years and Fonseca's first opponent ranked higher than No 20.

"I was just focusing (on) my game, trying to put no pressure on myself, playing (against) a top-10 guy," Fonseca said. "I was just playing my game and, well, now we're in the second round."

He'll next face 55th-ranked Sonego, who got past three-time Grand Slam champion Stan Wawrinka in four sets earlier Tuesday.

Asked the key to his strong play in the tiebreakers against the far-more-experienced Rublev, Fonseca said he tries to employ "all the intensity in the important points", "go for the shots" and show "courage".

He also cited 20-time major champion Roger Federer as an inspiration.

"Like Roger says: 'Talent is not enough.' ... So I put (in) a lot of hard work," Fonseca said. "Just me and my team knows that."

Agencies