Platten, who's set to tour with Colbie Caillat and Christina Perri later this summer, sounded confident in the stadium setting, flexing her bicep alongside Swift and taking a photo with the superstar at the end of the one-song performance.
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Meanwhile, Swift passionately co-delivered Platten's song to the audience before warmly embracing her new friend onstage.
"Fight Song" rises 11 spots to No. 25 on this week's Hot 100 chart. Last week, Swift posted an Instagram video of herself and Platten performing the "Fight Song" chorus backstage on a couch, with Kelsea Ballerini sitting next to them.
At Friday night's show in Philadelphia, Swift brought out Echosmith to perform their hit "Cool Kids" together. Mariska Hargitay was also on hand on Saturday night to take the catwalk during Swift's performance of "Style."