The first huge upset of the Australian Open has seen Stefanos Tsitsipas get smashed off the court and out of the tournament in a stunning four-set loss to unseeded American star Alex Michelsen.

Tstisipas lost 7-5 6-3 2-6 6-4 at John Cain Arena after stamping himself as a fan favourite as he stormed to last year's men's singles final. 

The world No 12 staged a brief comeback in the third set, but Michelsen - who is ranked 42nd on the ATP tour - wrestled the match back in the deciding stanza to record one of the biggest wins of his career.

It's the second straight first-round exit at a grand slam for the Greek star, who fell at the first hurdle in last year's US Open when he lost to Australia's Thanasi Kokkinakis in four sets.

The victory is the first time Michelsen, 20, has beaten a player ranked in the top 20 at a grand slam.

The deciding fourth set was a memorable one, with the pair trading outrageous winners along with untimely errors, the most costly of which was a double fault from 11th seed Tsitsipas on break point in the ninth game. 

A clearly shattered Tsitsipas still managed to share a warm embrace with the American at the net after match point.

'First of all, I just tried to stay super composed out there,' said Michelsen, who struggled at times with his second serve in the deciding set.

'I knew it was going to be a battle out there to the end.'

It was the first time since 2018 that Tsitsipas had lost in the first round at Melbourne Park.

He lost the 2023 final to Novak Djokovic and had also reached the semi-finals at Melbourne Park on three other occasions but is still searching for that elusive maiden major title.

Michelsen's second-round opponent will be either Australian wildcard James McCabe or Spanish qualifier Martin Landaluce.

Later in the day session on Monday, world No.1 Jannik Sinner starts his title defence against Chilean Nicolas Jarry.

The two feature matches of the night session involve 24-time major winner Novak Djokovic and No.3 seed Carlos Alcaraz.

In a result that went as expected on Monday, Coco Gauff continued her recent red-hot run of form, beating fellow American and 2020 Australian Open champ Sofia Kenin 6-3 6-3 to power into the second round at Melbourne Park.

Reigning French Open champion Gauff was not at her absolute best on Monday.

But she was still too good for world No.74 Kenin, who is a shadow of the player who won her maiden grand slam title at Melbourne Park in 2020, then reached the final of the French Open later that year.

She had also beaten Gauff in two of their previous three meetings - both at majors - but was unable to match it with the No.3 seed in the key moments on Monday.

Gauff, 20, has won her past 10 matches in succession and is riding a 19-2 win-loss run dating back to late September, including title triumphs at the recent United Cup in Sydney, the WTA Tour Finals in Saudi Arabia and the China Open.