Taylor Swift and Gracie Abrams perform their Grammy nominated song 'Us.' in Toronto.

Oh hi, Gracie Abrams! The 25-year-old songbird joined Taylor Swift on her third night in Toronto for Saturday's acoustic set.

"This is always one of my favorite parts of the night," Swift said in an orange and pink gown at the top of her surprise set. "I was kind of thinking about what to do tonight. The Eras Tour has been going on for like a year and five or six months, a year and a half now. And so a lot of life can happen in the phase of that tour. Like art can happen in the course of that tour. And I think you'll remember the genius, brilliant young woman you saw tonight."

The crowd erupted as Swift embarked on a story explaining how they wrote their song nominated for a Grammy shortly after Abrams opened up for Swift.

"We had so many fun memories during that tour," the Eras Tour ringleader said while strumming her guitar. "Part of that was we were celebrating her getting nominated for best new artist at the Grammys. We went out, and we had like eleventy-hundred cosmos and went back to my house, and we just started writing a song, and the coolest thing ever has happened this week."

Swift explained how she talked to Abrams on the phone following the nominations.

"It was just screeching, it wasn't even words," she told the Rogers Centre. "It was just feral screeching for the entire call, and we were so excited about that, and we wanted to figure out a way to say thank you to you so we put together a little something, so will you please welcome to the stage the magnificent Gracie Abrams," she said, throwing her hands above her head to clap.

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The diamond platform revealed the songstress.

Abrams crooned the first verse and chorus until Swift chirped in with, "Looking at it now, it all seems so simple," from "Out of the Woods." Abrams sang harmony as 49,000 fans belted the "1989 (Taylor's Version)" song.

Abrams in a white gown hugged Swift tightly and then bowed before jaunting down the catwalk to her exit point in the middle of the diamond.

Fans predicted the surprise appearance when Abrams didn't perform "Us." in her opening set. She's belted the number at every other show as the second to last tune in her set list.

The two debuted the song at the Eras Tour on June 23 shortly after the 25-year-old's album "The Secret of Us" came out. Their performance was the same night as Travis Kelce's famous stage appearance in Wembley Stadium.

On the piano, Swift performed a mashup of "You're On Your Own, Kid" from "Midnights" with "Long Story Short" from "Evermore."

Swift has a few days off and is halfway through her six Toronto concerts. Her final curtain will fall on Dec. 8 in Vancouver.

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This article originally appeared on Nashville Tennessean: Taylor Swift, Gracie Abrams perform their Grammy nominated song 'Us.'