The Shake It Off hitmaker has been delighting concert-goers with a slew of special guests at recent shows, performing with The Weeknd and Nick Jonas at gigs in New Jersey over the weekend (10-12Jul15), and introducing Lorde to the stage to belt out her track Royals on Monday (13Jul15), during the first of two dates in Washington, D.C.

On Tuesday, Swift turned up the heat as she returned to the stage at Nationals Park by bringing out Derulo to sing his tune Want To Want Me.

During the unexpected duet, Derulo sent fans screaming by stripping off his shirt to reveal his muscles, prompting Swift, who is dating Scottish DJ Calvin Harris, to squeal in delight.

After the show, she shared her appreciation for Derulo on Twitter.com, writing, "I got to sing my fav (favourite) summer JAM 'Want To Want Me' with @jasonderulo in DC-I'm also loving his new single 'Cheyenne'!"

Derulo also took to Twitter on Wednesday (15Jul15), and wrote: "Last night was so fly @taylorswift13 ! Thank u 4 having me join the party luv (sic)!"