Watch Taylor Swift Bring Out Rachel Platten for 'Fight Song' in Philadelphia

Taylor Swift
Jason Merritt/LP5/Getty Images for TAS

Taylor Swift performs during The 1989 World Tour live at Heinz Field on June 6, 2015 in Pittsburgh, Pennsylvania.

Less than a week after performing "Fight Song" with Rachel Platten in front of one other person, Taylor Swift and the rising singer-songwriter played it in front of 50,000.

Taylor Swift No. 1, Florence + The Machine & Jason Derulo Hit Top Five on Billboard Artist 100

On Saturday night (June 13) at Lincoln Financial Field in Philadelphia, Swift made Platten her special guest midway through the 1989 world tour performance with a rendition of the Top 40 hit "Fight Song."

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.

Taylor Swift Shares Zendaya's High School Graduation Pic: 'I Cry Endless Tears of Pride'

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."