Barlow is to play the second of two dates at the Royal Albert Hall. He was joined by Olly Murs, Jason Donovan and Lulu for last night's performance.
X Factor finalist Collins was due for a guest slot alongside Barlow, but had to pull out in order to rest his voice. However, the two will now pair up for tonight's show.
The Liverpudlian will join Amelia Lily and Little Mix in Saturday and Sunday's final. The three remaining acts appeared at BBC studios yesterday morning to speak to Radio 1's Chris Moyles.
Collins was wearing a badge saying "I am on voicerest" at the station to explain his quietness on the show.
He is currently scheduled to play a concert and make public appearances in Liverpool on Wednesday.
New tickets for Gary Barlow's show have just been made available to purchase via the Royal Albert Hall website.
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