In 2008, she competed at the Voronin Cup in Moscow, placing 3rd in the all-around and in the vault, floor exercise, and uneven bars finals.
In February, Demyanchuk competed at the 2009 WOGA Classic in Frisco, Texas, where she placed 3rd in the all-around, 1st on uneven bars, 2nd on floor exercise, and 4th on the balance beam.
Demyanchuk had her breakout competition at the 2009 European Championships in April, winning the balance beam title ahead of Olympians Anamaria Tămârjan and Gabriela Drăgoi of Romania.
At the 2009 World Artistic Gymnastics Championships in London in October, Demyanchuk competed with Ukrainian teammates Dariya Zgoba, Valentina Holenkova, and Anastasia Koval, but performed poorly.
She placed 10th on beam in the preliminary round and qualified to the event final, where she fell on her dismount and finished 8th with a score of 13.733.