[10] In January 2017, Kostyuk won the girls' singles title at the Australian Open, defeating Rebeka Masarova in the final.

[11] In May, she won an ITF tournament in Dunakeszi (Hungary) without dropping a set, becoming the youngest Ukrainian to win a professional singles title.

By defeating Peng Shuai in the first round, Kostyuk became the youngest player to win a main-draw match in Melbourne since Martina Hingis in 1996.

Kostyuk won the Burnie International, a $60k tournament in Australia, in February 2018, and reached the final of the Zhuhai Open in March, also a $60k event, but did not sustain her level of success in the rest of the year.

In 2019, she won two further ITF Circuit titles, and reached the quarterfinals of the WTA Tour event at Strasbourg as a qualifier where she lost to fourth seed Caroline Garcia.

[18][19] At the WTA 500 Abu Dhabi Open, she reached the semifinals defeating Lucie Hradecká, Hsieh Su-wei, Tamara Zidanšek and Sara Sorribes Tormo.

[22] At the WTA 500 Adelaide International 1, she went through qualifying and on to the quarterfinals, beating reigning Wimbledon champion, Elena Rybakina, en route.

In doubles at the same tournament, she reached the semifinals partnering Elena-Gabriela Ruse where they lost to eventual champions Barbora Krejčíková and Kateřina Siniaková.

[citation needed] At the Dubai Championships, after receiving a wildcard, she lost in the second round to eighth seed Belinda Bencic in the second longest match of the season, in three hours and 27 minutes.

[24] She reached her third quarterfinal of the season at the inaugural ATX Open in Austin, Texas defeating Dalma Gálfi and Madison Brengle.

[29] At the Miami Open, Kostyuk won her first-round match against Elisabetta Cocciaretto but was defeated by Anastasia Potapova in the second, and once again refused to shake her opponent's hand.

[31][1] Partnering Barbora Krejčíková, Kostyuk won the doubles title at the Birmingham Classic, defeating Storm Hunter and Alycia Parks in the final.

[32] At the Australian Open, she defeated Claire Liu, 25th seed Elise Mertens, Elina Avanesyan and another Russian, qualifier Maria Timofeeva, to reach the quarterfinals of a major for the first time.

It marked the first time multiple Ukrainian women have reached the third round, and also a record-breaking number of two of those players made it to the quarterfinals at a Grand Slam event in the Open era.

At the start of the clay-court season, at the WTA 500 Stuttgart Grand Prix, she defeated local wildcard Laura Siegemund, and fifth seed Zheng Qinwen, saving five match points to reach the quarterfinals.

In doubles at the same tournament, she reached the semifinals for the first time at this major, partnering again Elena-Gabriela Ruse, with wins over ninth seeds Leylah Fernandez / Erin Routliffe, and then Mirra Andreeva / Vera Zvonareva by walkover.

[43] Partnering Dayana Yastremska, Kostyuk represented Ukraine at the Paris Olympics, losing in the second round to Taiwanese pairing Hsieh Su-wei and Tsao Chia-yi.