Alex Michelsen produced the biggest win so far of his fledgling career to upset 2023 runner-up Stefanos Tsitsipas in the first round of the Australian Open, and he knew instinctively where credit was due. The 20-year-old American overcame nerves on his serve in the fourth set before clinching a 7-5, 6-3, 2-6, 6-4 win Monday over Tsitsipas, a 26-year-old from Greece who has a career-high No. 3 ranking and has contested two Grand Slam finals. Michelsen started playing tennis around age 3 and hit most days as a kid with his mother, Sondra, a school teacher who played college tennis.