The former Hollyoaks star and Fortuna had earlier been awarded a perfect 50/50 for their American smooth and 42/50 for their Argentine tango.
Craig Revel Horwood described their waltz as "Fab-u-lous", while Bruno Toniolo proclaimed: "The ballroom dazzler is back." However, Bastian was warned by judge Len Goodman that her nerves had been apparent during the tango.
"If you want to be here next week kissing that mirror ball trophy you have to kiss goodbye to your nerves because I think they really inhibit your performance," he said.
Hearing the result, the 26-year-old said: "We are going out on the most tremendous high and it has been the most incredible journey and I have loved every minute of it."
Bastian, who overcame an ankle injury to take to the floor, thanked fans at home for their support, before giving Fortuna a peck on the lips as she voiced her immense gratitude to him. Host Bruce Forsyth described the evening as the "best semi-final" in Strictly history.
The departure leaves presenter Chris Hollins and Hollyoaks star Ricky Whittle in the running to take the crown in next week's grand final.