Watch: Gracie Abrams joins Taylor Swift at Eras Tour to play their ...

Taylor Swift played a two-day old song during her surprise set on the guitar before shocking Wembley Stadium with an appearance from Gracie Abrams.

"If you know the words to this one, you get extra credit points, because it's only been out for like two days," Swift said at the start of her acoustic set in London on Sunday.

"This song is not technically mine at all. It's my friend Gracie's," Swift added in her pink dress.

She sang the first verse and chorus of "Us" before speaking again to the crowd of 89,000 fans.

"It's missing sort of the magic like a big, huge magical piece is missing," Swift said. "I just wish the actual artist who sings that song was here. If only Gracie Abrams were here. Hey, Gracie, you've got to come out and see this."

A gap in the diamond part of the stage appeared and Abrams walked down the catwalk in a gold dress before sitting at the piano.

"I love you so much," she said before explaining their songwriting process of drinking cosmos and wine. While the two wrote the song in Swift's Tribeca apartment, a small fire from a candle that had toppled over broke out and the Eras Tour star had to put it out with a fire extinguisher.

Swift said working with Abrams was magic.

"When you write with someone you see the whole thing," she said. "You see all the artistry. You see who this person is really, and writing with her, she's just so bad a**. That's a writer. I was telling her that I would gauge the excitement for her here in London."

Fans inside the stadium should have known something was afoot when Abrams left the star-studded VIP tent an hour into the show and never returned. Abrams' album "The Secret of Us" came out on Friday.

Abrams left the stage and Swift sat down on the piano to sing a mashup of "Out of the Woods," "Is It Over Now?" and "Clean" from "1989 (Taylor's Version)."

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