Video: Katy Perry Joined On Tour By Special Guest

Watch Katy team up with 'Friday' singer Rebecca Black.

Katy Perry is currently performing across America on her 'California Dreams' tour, and was joined on stage by YouTube star Rebecca Black.

Rebecca, who was made famous thanks to her 'Friday' track, was lucky enough to star in Katy Perry's music video for 'Last Friday Night (T.G.I.F.)'.

And now it seems Katy is such a fan she invited Rebecca to perform alongside her in LA.

Check out the video of Katy and her new music mate Rebecca collaborating on 'Friday'...