Oh, this is totally what we came for.

At Taylor Swift's only concert for the year, she surprised the audience at Circuit of the Americas in Austin, TX 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 helped write under a pseudonym and lent her vocals to, for a crowd she said totaled 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 in the future after This Is What You Came For was released. So, when Swift's rep confirmed her involvement with the hit, Harris unleashed his fury in a tweetstorm for the ages.

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