2014–2015 On the
ITF Junior Circuit, Fett had a career-high ranking of No. 12, which she achieved on 24 February 2014. She was the runner-up at the
2014 Australian Open girls' singles event, wherein she fell to
Elizaveta Kulichkova in the final. Fett's biggest title to date was at the
2015 Dunlop World Challenge, where she won the singles title defeating
Luksika Kumkhum in the final.
2017: WTA Tour debut, two semifinals, and top 100 At the
2017 Hobart International, she qualified for her first main draw of a
WTA tournament. She reached her first WTA Tour semifinal but lost to eventual champion and fellow qualifier,
Elise Mertens. Later in the year, she reached her second semifinal at the
Japan Women's Open, again coming through qualifying, while also scoring her first victory over a top-20 player, the top seed
Kristina Mladenovic. She lost to fellow qualifier
Miyu Kato, after failing to convert a match point. As a result, she made her top 100 debut on 18 September 2017.
2018: Major debut and first win At the
2018 Australian Open, she appeared in the main draw of a Grand Slam tournament for the first time as a direct entry and recorded her first major win over
Misa Eguchi. In the second round, she played against second seed
Caroline Wozniacki and had two match points, leading 40/15 at 5–1 in the third set. However, she lost that game and the successive five games to yield the match to the eventual tournament champion.
2022-2024: WTA 125 final, Roland Garros & WTA 1000 debuts She qualified for the main draw of the
2022 Wimbledon Championships after three years of absence at the All England Club where she lost to world No. 1,
Iga Świątek. She reached the final at the WTA 125
2023 Midland Tennis Classic but lost to first-time WTA Challenger titlist
Anna Kalinskaya. In April 2024, Fett won the biggest title in her career at the W100
Oeiras Open. On her debut at the
French Open, she entered the main draw as a lucky loser and recorded a first round win over
Jéssica Bouzas Maneiro, then losing her next match to
Marie Bouzková. At the
Jasmin Open, Fett reached the second round defeating
Mayar Sherif before losing to
Sara Sorribes Tormo in three sets. The following week at the
2024 Thailand Open 2, she reached the quarterfinals, after defeating
Zheng Saisai, and eighth seed
Varvara Gracheva by retirement, before going out to eventual champion
Rebecca Šramková. She entered the main draw of the WTA 1000
China Open as a lucky loser making her debut at this level, and recorded her first WTA 1000 win over wildcard entrant
Wang Meiling. Fett lost in the second round to ninth seed
Daria Kasatkina.
2026: Anti-doping suspension In January, the
International Tennis Integrity Agency (ITIA) announced that Fett had been provisionally suspended since 22 December 2025, after testing positive for three prohibited substances, namely
ostarine,
GW501516 and
LGD-4033, while representing Croatia in the BJK Cup on 16 November 2025. ==Grand Slam performance timeline==