The Rolling Stones brought out another special guest during a show on their 50th anniversary tour.

Taylor Swift joined the rockers on stage at Chicago's United Center on Monday night to sing the classic As Tears Go By. The ballad was one of the first songs written by lead singer Mick Jagger and guitarist Keith Richards. It was originally released as a single by singer Marianne Faithfull in 1964, but The Rolling Stones recorded its own version a year later.

"I'm going to slow it down a bit for you now to a bit of a ballad," Jagger said before the performance, also noting that the band hadn't performed the song in a while.

Swift, who was applauded as she walked on stage, couldn't conceal her excitement after performing with one of the most popular rock bands of all time.

"Filing this under 'never in my wildest dreams'. Thank you @RollingStones for inviting me to Chicago to sing with you," she tweeted on June 4.

That same day the singer - who performed with the band in a short black outfit, also tweeted a photo with Jagger with the caption, "Filing this under 'friends forever. ' "

Both were retweeted by the band's Twitter page.

The Rolling Stones have performed with several other guests during their '50 & Counting Tour. Keith Urban and Gwen Stefani joined the band to sing Wild Hoses and Respectable at the Staples Center in Los Angeles on May 3. Katy Perry sang Beast of Burden with Jagger on May 11 at the MGM Grand Garden Arena in Las Vegas; former American Idol winner Carrie Underwood also made an appearance at the Air Canada Centre in Toronto to sing It's Only Rock N' Roll (But I Like It) with the band.

The tour will continue until July 13. See the remaining dates on the band's website.

Tags: Taylor Swift Rolling Stones