Performing two songs each and signing autographs for fans and competition winners, it was the first time in the competition that the acts have performed outside of the Saturday night live shows.
Collins performed his versions of 'Reet Petite' and 'Superstition', before Little Mix took to the stage and recreated their take on En Vogue's 'Don't Let Go (Love)' and Katy Perry's 'E.T.'.
Bryan performed Tina Turner's 'Proud Mary', while Lily ended the evening with versions of Kelly Clarkson's 'Since U Been Gone' and Queen's 'The Show Must Go On'.
The semi-finalists revealed in a Q&A session that they had all spoken to Simon Cowell during the competition and that he had been offering them "constructive criticism" about their performances.
All four acts will battle it out for a spot at the Wembley Arena final this weekend. The semi-final kicks off at 8pm on ITV1.