Harry Styles and his new flame Taylor Swift belted out a karaoke duet after One Direction capped their journey to stardom with a huge gig in front of thousands of US fans at Madison Square Garden.
The pair performed Islands In The Stream, a Kenny Rogers and Dolly Parton classic, at the New York after-party, The Sun reported.
A source said: "It was brilliant. There was a huge cheer when they got up."
One Direction's rider was limited to chocolate treats such as Snickers and KitKats because of the US drinking age limit of 21.
Earlier Niall Horan told the crowd: "This is the best night of our lives. You have travelled from all around the world and we cannot believe what has happened here tonight. Thank you so much."
The band's album Up All Night was the first debut release by a British group to go straight in at number one in the US, and their second album Take Me Home shot to the top of the charts in 35 countries.