At the $25k tournament in Wellington, New Zealand, held in November 2010, Kostova, ranked 254 in the world, achieved her greatest victory in her career.

[2] Kostova won her first match in a WTA Tour main draw by beating Alizé Lim at the 2014 Bucharest Open, after coming through the qualifying competition.

They lost to defending champions, Irina-Camelia Begu and María Irigoyen, in straight sets.

In July 2016 came her biggest win when she won the final of the Europe Tennis Center Ladies Open in Budapest against fellow Fed Cup teammate Viktoriya Tomova.

[3] She retired in 2021 with her last match played at the Belgrade Challenger losing to eventual finalist Arantxa Rus.