The pair will perform their new track 'Goin' Crazy' together next Saturday (June 22).
It was previously announced that Michael Bublé will perform on the show's final. The Canadian singer will sing two tracks, with one song being made available to watch only on the BBC website.
Matt Henry, Leah McFall, Andrea Begley and Mike Ward won the public vote on tonight's semi-final and will compete against one another to win a recording contract next week.
McFall is widely tipped by bookmakers to win the show, after her rendition of 'I Will Survive' stormed the UK iTunes chart last week. It is expected to chart in the Top 10 on tomorrow evening's (June 16) UK Top 40 rundown.
will.i.am has described his artist as a "freak of nature", adding that he thinks she is better than fellow coach Jessie J.
The Voice UK grand final airs next Saturday at 7.15pm on BBC One.
Photo gallery - The Voice UK semi-final: