All five remaining acts - Mary Byrne, Cher Lloyd, One Direction, Matt Cardle and Rebecca Ferguson - will perform two songs this weekend and one of the tracks will have to be a popular dancefloor hit.
For their second track, the acts will pick their own song - one which they believe can get them into the final.
Speaking on Matt Edmondson's F Factor blog about the themes, vocal coach Yvie Burnett said: "This is the most important song choice so far. We have got absolutely every type of act you want in this semi-final.
"Everyone is getting really stroppy, really grumpy, really tense and nobody can decide on their songs. This week is crucial."
The X Factor semi-final airs on Saturday at 7.40pm on ITV1.