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 defeat a top-11 seed in the opening round in Melbourne, joining Patrick McEnroe, who defeated Boris Becker in 1995.

Michelsen played with freedom against Tsitsipas, taking big swipes with his service returns — including three in one game late in the fourth set that helped earn him a vital break.

He got a bit tense on serve, surrendering two hard-earned breaks in the fourth set, but stayed composed in the last game. He finished the match with eight aces and eight double-faults, but hit 46 winners to only 40 unforced errors.

“First of all, I was just trying to stay super composed out there. I knew it was going to be a battle in the end,” he said. The serve “started to let me down a little bit in the fourth, but super happy. It’s all about the mindset.”

Tsitsipas also lost in the first round of last year’s US Open.

“My whole role was to try to go deep into the Australian Open. I knew the first thing I had to consider was not play doubles,” Tsitsipas said. “I guess karma hit me, I was not able to deliver or play the way I was hoping to … The whole purpose was just to save up on some energy and be fresher hopefully in the deeper draw of the tournament.”

ESPN Research, The Associated Press and Reuters contributed to this report.