Gauff off to a flyer before Djokovic, Sinner begin Open quests
Novak Djokovic begins his bid for Grand Slam history later on a blockbuster day two and Jannik Sinner starts the defence of the trophy he won 12 months ago.
Carlos Alcaraz also gets under way at the first Grand Slam of the year as he looks to lift the one major title to elude him.
In the women's draw Iga Swiatek and two-time Australian Open Champion Naomi Osaka are both in action.
In-form Gauff laid down a marker as she swept aside former champion and fellow American Sofia Kenin in straight sets on Rod Laver Arena.
The third seed, who is unbeaten this year after leading the US to win the United Cup, eased past the 2020 Melbourne Park winner 6-3, 6-3 in 80 minutes.
"I knew it was going to be difficult, but you know, I'm happy with how I played," said Gauff, the 2023 US Open champion.
The 11th-seeded Tsitsipas crashed out at the first hurdle when young American Alex Michelsen blasted past him 7-5, 6-3, 2-6, 6-4 on John Cain Arena.
The 26-year-old Greek, who played Novak Djokovic in the 2023 final at Melbourne Park, never looked comfortable against the 20-year-old Michelsen who had him running all over the court.
"I just tried to stay super composed out there today, I knew it was going to be a battle to the end," said the 42nd-ranked American, who scored his first win over a top-20 player at a Slam.
Serbian great Djokovic is hoping to roll back the years and win an 11th Australian Open and record 25th Grand Slam singles title.
He faces American Nishesh Basavareddy in the prime-time evening match on Rod Laver Arena.
"To be honest, I don't know much about him," said Djokovic, who is now coached by former rival Andy Murray.
"I'm sure that he is going to be really pumped to make a statement."
'Good to be back'
He will be preceded on court by world number one Sinner with the Italian aiming to put a doping scandal behind him when he opens his title defence against Chile's Nicolas Jarry.
Sinner is under a cloud after twice testing positive for traces of the steroid clostebol in March.
While exonerated by the International Tennis Integrity Agency, the World Anti-Doping Agency appealed and his case which will be heard by the Court of Arbitration for Sport on April 16-17.
Spain's Alcaraz is bidding to complete a career Grand Slam of all four majors aged just 21 and begins against in-form Kazakh Alexander Shevchenko in the night session on Margaret Court Arena.
The four-time major winner Alcaraz has never gone past the quarter-finals in Melbourne.
Women's second seed Swiatek has been shunted out to John Cain Arena for what appears to be a straightforward opener against Czech doubles specialist Katerina Siniakova, ranked 46.
Japan's Osaka will wrap up the night session on centre court against France's 58th-ranked Caroline Garcia in a repeat of last year's first-round clash that the French player won.
Now ranked 50th, Osaka won two of her four Grand Slam titles to date at Melbourne Park in 2019 and 2021.
She reached her first final since 2022 in Auckland eight days ago, only to retire injured.
"I've been practising pretty well ... so it seems to be going good," she said.
Australian fans will be treated to seeing their controversial hero Nick Kyrgios in action.
The polarising local hope made his return to tennis after 18 months last week in Brisbane following knee surgery and wrist reconstruction.
He faces Britain's Jacob Fearnley in an evening match on his favoured "party court", John Cain Arena, where he is promising fireworks.
"I think it's good to be back. I think it's important," said the 29-year-old.
"I think the sport was getting a bit mundane."