Surprise! In her first post-baby singing performance, Kelly Clarkson made a surprise appearance at country superstars Garth Brooks and Trisha Yearwood's concert in Chicago on Sept. 5. Clarkson hasn't performed since giving birth to her daughter, River Rose Blackstock, in June.

In this fan video released online (courtesy of Just Jared), Brooks says to Yearwood, "You've got this cool new song coming out. It's with you and Kelly Clarkson." After a roar of cheers from the crowd, Brooks tells everyone, "Hey! I'm going to be Kelly Clarkson tonight! I can sing better than Kelly Clarkson."

But not so fast, Garth! Clarkson walks on stage and joins Yearwood in a performance of their song 'Prizefighter.' 'Prizefighter' is Yearwood's first song in six years and is an inspirational country ballad about standing up after getting knocked down. The lyrics read, “When you hit the ground / You find your wings / You go one more round / When that bell rings / You say you’re done but here you come / You’re a hammer hitting, spitting fire / Prize fighter.”

After the show, Clarkson tweeted about her experience and also shared an adorable photo of River at her first concert ever!

Click on the video to watch Clarkson and Yearwood duet on 'Prizefighter.'