Watch: Taylor Swift sings ex Calvin Harris' 'This Is What You Came For'

AUSTIN — Oh, this is totally what we came for.

At Taylor Swift's only concert of the year, she surprised the audience at Circuit of the Americas race track by singing ex-boyfriend Calvin Harris' This Is What You Came For, which features Rihanna.

Swift, seated at a bedazzled piano, played the song that she co-wrote under a pseudonym for a crowd of 80,000.

"I've never played this song live before," Swift said, stroking the keys. "But if you know it, maybe you could sing along, and then I could get to know what that feels like to hear it sung back to me."

The singer is credited under the name Nils Sjöberg, but Harris tweeted (in a series of raw tweets) that Swift wanted her contribution to the single kept under wraps. The DJ went so far as to tell Ryan Seacrest in an interview that he couldn't see a collaboration with Swift happening. So when Swift's representative confirmed her involvement with the hit, Harris unleashed his fury in a tweetstorm for the ages.

But going back to her show, here are other moments any Swifties would be excited to know:

1. Tay-Tay had to take a tissue break because of her cold

Just like Beyoncé lets out a sneeze every now and then, Swift gets sick. She revealed during her performance that she caught a cold three days ago and even had to take "a tissue break." "I promise it'll sound like 40% better if I do this," she told the crowd.

2. She acknowledged her 10-year anniversary

Swift released her self-titled CD on Oct. 24, 2006, and reflected on her decade in the music business.

"It's been about 10 years since my first album came out when I was 16," she began. "And since then, I felt like I've been opening up my journal and letting you read it. And it's so amazing to know that 10 years later, you still feel like reading it, and you still feel like coming out and being here tonight."

Later, she added, "In the last 10 years, my life has changed so much, but even with all of those changes, even with everything that has happened and just how you move to different places, you experience different things, you travel places you never thought you'd get to, whatever. If I were to pick one year of my life where I learned the most ... I would still say that it was when I was 15." And then, of course, she played Fifteen.

3. In honor of her Swiftiversary, she played some older hits

While she didn't play or mention any new music, Swift did include familiar favorites in her lineup such as You Belong With Me, Love Story, Sparks Fly and Enchanted.