Fearless teenage qualifier Joao Fonseca beat ninth seed Andrey Rublev on his Grand Slam debut on Tuesday in the biggest shock of the Australian Open so far. 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 (7/1), 6-3, 7-6 (7/5). 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 27-year-old 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 tie break. 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.