A source close to the situation explained:
Katy Perry Blows John Mayer Fans Away With Surprise Concert Drop-In! Watch Them Perform Who You Love Live HERE!
We're guessing by the audience reaction that they did NOT expect to be hearing the steamy new song Who You Love on Tuesday night at the Barclays Center!
Or if they did, it that it would just be John singing it to himself. Weird, but still kinda sexy… LOLz!
See the adorable couple duet …AFTER THE JUMP!!!
[Image via YouTube.]