MELBOURNE, Australia (AP) — Coco Gauff had a little difficulty adjusting to the sun at one end of Rod Laver Arena and dropped an early service game but quickly settled into a rhythm to start her Australian Open with a 6-3, 6-3 win Monday, Jan. 13, over 2020 champion Sofia Kenin.

Third-seeded Gauff won the title at the WTA Finals last November and started this season by helping the U.S. to victory at the United Cup last week, a run that gives her a chance to move atop the rankings.

One of her potential obstacles to that No. 1 ranking, No. 2 Iga Swiatek, a five-time major winner from Poland, fended off top-ranked doubles player Katerina Siniakova, 6-3, 6-4.

Swiatek, who lost to Gauff at the WTA Finals and the United Cup, served a one-month ban for a doping infringement late last year and it has been a big talking point at the Australian Open.

Gauff, the 2023 U.S. 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. 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.

Guaff will next play Jodie Burrage of Britain. Also advancing from early matches were No. 12 Diana Shnaider, No. 23 Magdalena Frech, No. 25 Liudmila Samsonova and No. 28 Elina Svitolina. Lucia Bronzetti of Italy ousted No. 21 Victoria Azarenka, a two-time Australian Open champion.

After a stop-start Day 1 on Sunday, including more than six hours of rain, Monday’s packed program also includes matches for 10-time Australian Open champion Novak Djokovic, defending champion Jannik Sinner, Carlos Alcaraz.