Two-time Grand Slam runner-up Stefanos Tsitsipas said "it sucks" after being on the end of the first big upset of the Australian Open on Monday, with the worst part having to hang around before his next tournament.

The 11th-seeded Greek, who played Novak Djokovic in the 2023 final at Melbourne Park, crashed out 7-5, 6-3, 2-6, 6-4 to American Alex Michelsen in the first round.

"The most frustrating part about losing in the first round of a Grand Slam is that you have way too much time to recover, and I would rather have the other way around where I don't have enough time to recover," he said.

"Honestly that's much, much better in terms of problem-solving. It just sucks in a way that I'll be hanging around for quite a while now before my next tournament comes in. With my competitive nature, I feel like these type of things are not really ideal for me."

Tsitsipas never looked comfortable against the 20-year-old, 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 American, ranked 42, who scored his first win over a top-20 player at a Slam.

"My serve sort of let me down a bit in the fourth set, but super happy to get through it. It's all about the mindset. I came in with the proper mindset and I executed the game plan," he added.

Defeat compounded Tsitsipas's problems after a mediocre 2024 in which his ranking dropped to its current 12 from a career-high three.

He managed only one title, a repeat victory at Monte Carlo, and also suffered a first-round exit at the U.S. Open.

Tsitsipas was hoping for a reset this year, but his season started badly when he was beaten in his opening match at the United Cup by 77th-ranked Alexander Shevchenko and he carried that form to Melbourne.

"It was a difficult first-round match. I knew I was dealing with a pretty serious opponent because I've played him before, and I have lost," he said.

"I had a very slow start. In terms of finding my movements and just dominating from the serve plus one, it wasn't really the way I'm expecting it to work. That led to some frustration and, let's say, insecurity in terms of approaching my game.

"I just hope I'm able to retrieve that back and use that as a strength within my game."

Novak Djokovic begins his bid for Grand Slam history later on a blockbuster day two and Jannik Sinner starts the defense 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.

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."

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 defense 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 quarterfinals in Melbourne.

Australian fans will be treated to seeing their controversial hero Nick Kyrgios in action.

The polarizing 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."

In women's action, world number three Coco Gauff began her Australian Open campaign in style.

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 U.S. 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.

Iga Swiatek and two-time Australian Open Champion Naomi Osaka are both in action Monday.

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 center 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.