The 16-year-old singer, who has been tipped to win throughout the competition, said that he gets scared that people won't vote for him to win because they presume that he's safe.
Mastin told The Sunday Telegraph: "My biggest risk... is if people think I'm the favourite, they won't bother voting."
He also admitted that being slammed for his performance of Celine Dion's 'All By Myself' last week was a shock to the system, saying: "It was definitely a wake-up call. There's only four people left so it's really close and everybody is so good. Last week was pretty full on. It wasn't very fun."
Judge Ronan Keating caused controversy earlier in the series when he told Mastin to forget about his education in order to concentrate on his music career.
Mastin's rumoured girlfriend and former fellow contestant Christina Parie was eliminated from the competition two weeks ago. Johnny Ruffo, Andrew Wishart and Three Wishez still remain in the contest along with Mastin.
Watch Reece Mastin's performance of 'All By Myself' below: