Coco Gauff had a little difficulty adjusting to the sun at one end of Rod Laver Arena in Monday's first marquee match and dropped an early service game before quickly settling into a rhythm in a 6-3, 6-3 win over 2020 champion Sofia Kenin.

Third-seeded Gauff beat Swiatek to win the title at the WTA Finals last November and started this season by helping the US to victory at the United Cup last week, again beating No 2 Swiatek in the final.

Gauff, the 2023 US Open champion is wearing a Marvel-inspired bodysuit and skirt at Melbourne Park and she's radiating confidence and calm.

"I knew going in it was going to be difficult, but you know I'm happy with how I played," she said of the 1-hour, 20-minute win over Kenin, who at No 81 is a much tougher opponent than her ranking suggests.

"I mean (I) could serve better, but like on that side I was struggling to see the ball," Guaff said, pointing to one baseline on the main show court that was bathed in sun. "So I'm just happy that I was able to manage through that."

Another 20-year-old American, Alex Michelsen, produced the biggest win of his young career with a 7-5, 6-3, 2-6, 6-4 first-round upset of 2023 Australian runner-up Stefanos Tsitsipas.

Michelsen's three booming service returns in the ninth game of the fourth set helped him earn a vital break against the 11th-seeded Tsitsipas and, after shaking off jitters on his own serve, he closed out for his first victory over a player ranked in the top 20 at a Grand Slam.

Tsitsipas could only blame himself for his jaded performance at the Grand Slam where he once beat both Rafa Nadal and Roger Federer in their prime.

"I had a fresher mind back then," the 26-year-old said.

"I would definitely say that one thing that stood out the most back then is that I had this hunger to try and make a life out of tennis.... It was different from it is now."

No 17 Frances Tiafoe advanced in five sets over Arthur Rinderknech of France.

Gauff went into her opening round on a streak of winning 33 matches against players ranked outside the Top 50, dating to a loss to Kenin at Wimbledon 2023.

It was tough going early, with Gauff converting an early break before Kenin got back on serve at 2-2, hitting deep ground strokes and getting good bounce off the hard court.

But Gauff, who had five aces and four double-faults in her first two service games, lifted her tempo, started taking the ball earlier and won four of the next five games. She finished the match with 12 aces and nine double-faults, and saved seven of the eight breakpoints she faced. She also had 28 winners, including two rifling backhands on key points in the second set, and 13 unforced errors.

The draw presented another tough trip to Australia for Kenin. It was the third straight year Kenin faced a Grand Slam champion in the first round in Australia, and her fourth consecutive first-round exit here.

Twice champion Victoria Azarenka was sent packing by unseeded Italian Lucia Bronzetti, who beat the Belarusian 21st seed 6-2, 7-6 (2).

Gauff will next play Jodie Burrage of Britain. No 7 Jessica Pegula also advanced along with No 12 Diana Shnaider, No 28 Elina Svitolina, and Belinda Bencic, who ousted No 16 Jelena Ostapenko.

Agencies via Xinhua