WASHINGTON • At her only concert of the year, Taylor Swift threw a curveball at the audience.

At the Formula 1 United States Grand Prix in Austin, Texas, on Saturday, the singer sang her hits. But she also performed her ex-boyfriend Calvin Harris' This Is What You Came For, which she co-wrote using a pseudonym. It was the first time she sang it live.

"As a songwriter, the most rewarding feeling in the world is writing something and then having a crowd sing it back to you because they know the words. It's the coolest thing in the world," Swift told the crowd.

"I've never played this song live before. But if you know it, maybe you could sing along and then I could get to know what that feels like to have it sung back to me?" she asked, fans screaming as she launched into an acoustic piano version.

The single topped the charts this summer when it was released by Harris. It became roiled in controversy following their break-up and Swift revealing that she had co-written it after originally keeping it a secret.

Harris tweeted that the revelation was to make him look bad and Swift, who was then photographed making out with actor Tom Hiddleston, was tearing him down.

WASHINGTON POST