Roared on by Brazilian fans decked out in yellow, Fonseca appeared totally unfazed as he set up a second-round meeting with Italy’s Lorenzo Sonego.
“It’s the first time playing in a huge stadium,” he told an energised Margaret Court Arena.
“There are a lot of Brazilians here cheering for me and I enjoyed every moment, thanks a lot.”
Rublev’s fellow Russian Medvedev, a three-time losing finalist in Melbourne including a year ago to Jannik Sinner, was heavy favourite against Thailand’s 418th-ranked Kasidit Samrej.
But in his first match of the season, the fifth seed nearly imploded in a fit of anger before finding some measure of calm.
“Second and third set I couldn’t touch the ball. I didn’t know what to do,” Medvedev said after finally winning 6-2, 4-6, 3-6, 6-1, 6-2 on Rod Laver Arena.
The 28-year-old mangled his racquet as he spectacularly lost his rag in the third set, slamming it into the net camera repeatedly until they both became a broken mess.
There was no such trouble for fourth seed Fritz as he blitzed fellow American Jenson Brooksby 6-2, 6-0, 6-3 to launch his bid for a maiden Grand Slam crown.
“It’s never easy playing that first match in a Slam, there are some nerves, so I did a really good job shaking them off early and playing really solid,” said Fritz, who took just one hour and 46 minutes to dismantle his compatriot.
France’s Monfils battled through a five-set thriller to topple young countryman Giovanni Mpetshi Perricard and continue the 38-year-old’s late-career resurgence.
Crowd-pleaser Monfils became the oldest singles champion in ATP Tour history when he swept to victory at the Auckland Classic on Saturday.
He kept the momentum going against lethal-serving Mpetshi Perricard, 21, over a gruelling three hours and 46 minutes.
“I’m crazy happy,” said the popular Monfils.
“Everything is a win now, so, you know, no pressure on me.”
De Minaur, nicknamed “The Demon” by Australian fans and media, eased past Dutchman Botic van de Zandschulp, 6-1, 7-5, 6-4.
Italy’s Paolini breezed past Wei Sijia 6-0, 6-4 in just 73 minutes to underline her title credentials.
The 29-year-old has said she got a taste for playing on the biggest stage after rocketing to world number four last year on the back of finishing runner-up at both Roland Garros and Wimbledon.
“Maybe everything began from here last year,” Paolini, who reached the fourth round 12 months ago, said after hammering the world number 117 from China.
Former Wimbledon champion Rybakina dished out a harsh lesson of her own to 16-year-old Emerson Jones, winning 6-1, 6-1 in a ruthless statement of intent.
The sixth seed from Kazakhstan demonstrated her pedigree as she manoeuvred the world junior number one from Australia to all parts of Margaret Court Arena.
The 2021 US Open champion Raducanu blew hot and cold in beating 26th seed Ekaterina Alexandrova from Russia, 7-6 (7/4), 7-6 (7/2).
The 22-year-old Briton, who next faces American Amanda Anisimova, clocked up 15 double faults and said that her serve had “a mind of its own”.
Navarro, the eighth seed, beat fellow American Peyton Stearns in three sets.
The 13th seeding proved unlucky for Anna Kalinskaya, a quarter-finalist last year, who withdrew moments before her match against Australia’s Kimberly Birrell.
Kalinskaya was replaced by lucky loser Eva Lys of Germany, who took full advantage of her last-minute call-up by winning 6-2, 6-2.
Many of the top names including Novak Djokovic, Aryna Sabalenka, Carlos Alcaraz, Coco Gauff and Naomi Osaka will be back in action on Wednesday when the second round begins.
