Americans Alex Michelsen, Frances Tiafoe advance at Australian Open - ESPN

MELBOURNE, Australia — Alex Michelsen produced the biggest win so far of his fledgling career to upset 2023 runner-up Stefanos Tsitsipas, the No. 11 seed, in the first round of the Australian Open on Monday.

The 20-year-old unseeded American overcame nerves on his serve in the fourth set before clinching a 7-5, 6-3, 2-6, 6-4 win over Tsitsipas, a 26-year-old from Greece who has a career-high No. 3 ranking and has contested two Grand Slam finals.

Joining Michelsen in the second round will be fellow American Frances Tiafoe, who required five sets to get past France's Arthur Rinderknech 7-6 (2), 6-3, 4-6, 6-7 (4), 6-3.

Tiafoe, the No. 17 seed and a former Australian Open quarterfinalist, vomited at the end of fourth set before wrapping up the victory in 4 hours and 8 minutes. He advances to his fifth straight Australian Open second round.

Michelsen, who had never beaten anyone seeded higher than 32 at a Grand Slam, started playing tennis around age 3 and hit most days as a kid with his mother, Sondra, a school teacher who played college tennis.

«Yeah, I'm sure she's watching right now,» Michelsen told the crowd on John Cain Arena, one of the three main show courts at Melbourne Park. «Yeah, we hit a million balls from the baseline every day. We'd go like 30 minutes up the middle, then we go across each way for like an hour and a half.

»I mean we would just go out there and she would never miss a ball — she's incredible. But no chance I'd be here without without her, so thanks Mom. Love you."

The No. 42-ranked Michelsen reached the third round last year on his Australia debut before losing in the first rounds at Roland Garros and Wimbledon and in the second round at the US Open.

He became the second unseeded American man to