US Open champion Raducanu claimed her first ever regular WTA Tour win in Cluj earlier this week, with the 18-year-old keen to impress in the homeland of her father, but Kostyuk denied her the chance of a dream last-four clash with Halep.
The 19-year-old Ukrainian swept Raducanu aside in just 57 minutes, winning 6-2 6-1 as Kostyuk furthered her own burgeoning reputation in the game by reaching a third semi-final of the year.
Raducanu proved her own worst enemy, her 41 unforced errors more than double Kostyuk's 16, with the latter feeling confident as early as the first game when two double faults helped her break the Briton's serve.
"I don't know how I won the first game, I think that's where everything started," Kostyuk said. "I somehow won that game and I gained this confidence, like I'm not going to lose this match.
"After that, by the fifth game maybe, I figured out how to play her – today. Maybe next time I play it's going to be different tactics."
No.1 seed Halep enjoyed a similarly straightforward victory as she lost just two games en route to a 6-1 6-1 win over Romanian compatriot Jaqueline Cristian, a particularly impressive feat given she had been suffering with a back injury this week.
That success preserved Halep's 100 per cent record against fellow Romanians.
On the other side of the draw, No.2 seed Anett Kontaveit was also an impressive winner as she disposed of Anhelina Kalinina 6-1 6-3 in just over an hour. Kontaveit has won 24 of her past 26 matches and will be the firm favourite against unseeded Rebecca Peterson, who beat Lesia Tsurenko 6-2 3-6 6-3.
At the Courmayeur Open in Italy, No.3 seed Liudmila Samsonova cruised into the semi-finals thanks to a swift 6-1 6-2 demolition of Anna Kalinskaya. She will meet Clara Tauson, the No.5 seed, after the Dane came through a more gruelling 3-6 6-4 6-2 win over Ann Li.
The other semi-final will be contested by Donna Vekic and Jasmine Paolini, who were straight-sets winners over Wang Xinyu and Dayana Yastremska, respectively.