Collins and the Take That star were up first with their rendition of Billy Joel's 'Always A Woman', which as reported was done in tribute to the Liverpudlian contestant's mother.
Barlow, who played along on a piano throughout their performance, told host Dermot O'Leary afterwards that he had been most impressed by Collins's adaptability throughout the contest.
"Honestly, Marcus has been incredible," he said. "One of the great things about Marcus is he listens, he takes on board everything you say and comes back even better... he's incredible."
Collins responded by describing the duet as his "dream come true", adding: "This has been unforgettable...my mum was probably crying, she's probably hidden."
Next up were Contostavlos and Little Mix, who took on an Alicia Keys mash-up of 'If I Ain't Got You' and 'Empire State Of Mind', complete with some beatboxing from bandmember Jesy Nelson.
"[It's been] an amazing experience," Contostavlos said. "I really have met friends. [Little Mix are] such true, genuine, honest, talented young ladies who deserve this more than anyone I know. I love them from the bottom of my heart."
Little Mix singer Jade Thirlwall insisted to O'Leary that the group wouldn't have wanted to perform with anyone else and described the final as "the most amazing day of all our lives".
Finally, Rowland and Lily took to the stage for a lively cover of Tina Turner's 'River Deep, Mountain High', which the Destiny's Child star said demonstrated her act's status as a "born superstar".
"Listen to her, she is a born superstar from her voice to... everything about her," Rowland said. "She's so thankful to be here too, she's so incredibly humble and she's persevered so much. I'm so incredibly proud of [her]."
A clearly-emotional Lily added: "I couldn't ask for a better mentor. This lady has been the biggest person in my life right now and she is an incredible inspiration to me.
"For her to perform on this stage with me tonight is one of the best moments of my life."
Seemingly debunking reports that she and Lily had fallen out over her commitment to the show, Rowland ended the segment by pulling the teenager into a tight embrace.
With the duets at an end, remaining judge Louis Walsh declared that Barlow's performance with Collins had been the highlight of the night and complimented Contostavlos on her vocal abilities.