The teenage singer, who has previously been tipped as the favourite to win the reality television show, was criticised for his rendition of Celine Dion's 'All By Myself' on last night's 'Legends'-themed show.
After his performance, judge Ronan Keating told Mastin: "You were unsure about the song and I could see it in your eyes."
Keating, Mel B and Natalie Bassingthwaighte all blamed Mastin's mentor Guy Sebastian for choosing the wrong song for him.
Other contestants Johnny Ruffo, Andrew Wishart, Three Wishez and Declan Sykes were all praised by the judges for their performances.
Last week, Mastin found himself in the bottom two contestants with rumoured girlfriend Christina Parie, but she was later eliminated.
Mastin could now face elimination in tonight's live show.
Watch Reece Mastin's performance of 'All By Myself' below: