Niculescu entered the Grand Slam doubles final at Wimbledon 2017 tournament, together with Chan Hao-ching.

Her mother Cristiana Silvia is a pharmaceutical sales representative, and her father Mihai Niculescu is an engineer.

In February of the following year, she made her WTA Tour main-draw debut at the Dubai Tennis Championships.

A month later, at the Qatar Open, she made her Tier I debut and also recorded her first win on the WTA Tour in a singles event.

In doubles events, she managed to get to her first WTA final at the Connecticut Open alongside compatriot Sorana Cîrstea, but they failed to win the trophy.

In doubles, in February alongside Cîrstea, she reached semifinals of the Premier Open GdF Suez but then withdrew.

She then won a WTA Tour doubles title, at the Budapest Grand Prix, winning alongside Alisa Kleybanova.

She beat fellow Romanian and top-30 player, Alexandra Dulgheru, then lost her semifinal match to Elena Vesnina.

During the year, she recorded only one major match win at Wimbledon, defeating Gisela Dulko in the first round.

She then jumped to the final of the Luxembourg Open for the second year in a row after making four consecutive straight-sets wins.

In early March, she won her first WTA singles title at the Brasil Tennis Cup in Florianópolis, after defeating Olga Puchkova in the final.

Alongside fellow Romanian Irina-Camelia Begu, they ended runners-up, losing the final to Martina Hingis and Sania Mirza.

In singles, she advanced to the second round after defeating Verónica Cepede Royg but was then forced to withdraw due to injury.

[12] A month later, she won her third career singles WTA title at the Luxembourg Open after beating Petra Kvitová.

At Wimbledon, alongside Chan Hao-ching, she finished as a runner-up after losing to Ekaterina Makarova and Vesnina.

[16] At the Cincinnati Open, she reached the semifinal in doubles event alongside Hsieh; they lost to Hingis-Latisha Chan.

[18] Her 2018 season started with playing at the Hobart International; she advanced to the quarterfinals after two wins but was then forced to withdraw.

[19] Despite losing in the first round of the singles event at the Australian Open, she managed to reach semifinals in doubles, partnering with Begu.

The following week, she won the $100k Ilkley Trophy grass-court tournament, defeating Tímea Babos in the final.

At the 2020 Australian Open, Niculescu qualified into the main draw, facing Alizé Cornet in the first round.

At the Prague Open, Niculescu finished runner-up alongside Olaru losing to Lucie Hradecká and Kristýna Plíšková.

At the 2021 Qatar Open doubles event, she and Jeļena Ostapenko finished runners-up, losing the final to Nicole Melichar and Demi Schuurs.

[31] Next, she won the ITF title at the $100k Nottingham Trophy, where, together with Elena-Gabriela Ruse, she defeated Priscilla Hon and Storm Sanders.

[36] She ended the season with the doubles title at the WTA 125 Open de Limoges, alongside Vera Zvonareva.

[37] Partnering Irina-Camelia Begu, Niculescu represented Romania at the Paris Olympics, losing in the first round to Taiwanese pairing Hsieh Su-wei and Tsao Chia-yi.

[38] Alongside Elena-Gabriela Ruse, she won the doubles at the Hong Kong 125 Open in October, defeating Nao Hibino and Makoto Ninomiya in the final.

[39] Niculescu won the Monterrey Open doubles title with Guo Hanyu, after beating Giuliana Olmos and Alexandra Panova in the match tiebreaker.

[40] Alongside Elena-Gabriela Ruse, she won the doubles at the Hong Kong 125 Open in October, defeating Nao Hibino and Makoto Ninomiya in the final.

[39] Partnering Elena-Gabriela Ruse, Niculescu won the doubles title at the WTA 125 Open Angers Arena Loire, defeating Belinda Bencic and Celine Naef in the final.

[41] Partnering with Fanny Stollár, Niculescu reached the doubles final at the Hobart International, losing to Jiang Xinyu and Wu Fang-hsien.