Sykes, who has been debuting his solo material on his own sold-out tour dates across the country, was welcomed to the stage in Birmingham by the 'Dear Future Husband' star for a surprise performance of her track 'Like I'm Gonna Lose You', reported MTV UK.
John Legend duets with Trainor on the studio version of the song, which features on her chart-topping debut album Title.
Trainor shared a picture on Instagram, writing: "I loved singing with @nathansykes at my last show in #Birmingham I'm obsessed with his voice."
Last week, the former The Wanted singer confirmed that he will release his first solo single 'Kiss Me Quick' on June 14.