The great thing about seeing Taylor Swift live in concert is that you never know who she'll bring out on stage to share the microphone with her. On Monday (Aug. 19) at the Staples Center in Los Angeles, Swift gave fans a major surprise when she performed a duet with Sara Bareilles.

By the time Swift announced that Bareilles would be joining her, fans probably thought that they had seen all the special guests the 'Red' hitmaker was going to invite on stage that night. Ed Sheeran was on hand to perform his pop hits and team up with Swift on 'Everything Has Changed,' and Cher Lloyd performed 'Want U Back' with the country crossover star.

Later on, Swift took a break from dancing and singing her heart out to have a little chat with her audience. "I certainly don't think that you'd expect us to bring two special guests," she hinted, which brought on massive screams.

"There's a song that came out a few months ago, and I listened to it every time I had a bad self esteem day -- which is a lot," she continued. "This song is so beautifully written, it's so original. It's something I've listened to hundreds and thousands of times..."

Swift continued to sing the praises of this "mystery" artist before finally shouting: "Sara Bareilles is here!"

Bareilles walked confidently on stage and started belting out the first verse of her hit song 'Brave.' Swift danced alongside before chiming in for a duet at the chorus. By the second and third verses, Bareilles and Swift were trading back and forth, acting like they had sung 'Brave' together hundreds of times.

After Swift's show ended, the superstar logged in to Twitter to express her affection for Bareilles.

"Tonight at Staples was an absolute DREAM," Swift tweeted. "@SaraBareilles came out to sing 'Brave,' one of my favorite songs. And she completely smashed it!!"

Monday was the first of a four-night stint at the Staples Center for Swift's 2013 Red Tour. We can't wait to see what she does to wow her audience for the next three performances!