Madonna caused a sensation by inviting Psy to her concert as part of her worldwide tour at Madison Square Garden in New York on Wednesday (local time).

She performed the horse riding dance with Psy to his hit single "Gangnam Style," even at one point crawling beneath his legs -- a gesture he quickly reciprocated.

Rallying the crowd, Madonna, 54, said, "He flew in all the way from Frankfurt, Germany this morning just to cheer your asses up!"

She continued, "Thank you, Psy! I'm a very big fan of yours. I've never been out to Korea, but I'll have to do it one day." A delighted Psy responded, "That's so sweet. I have to say this: In the last couple of months I've experienced so many things, but now I'm at MSG with Madonna!"

While singing a remix of her 2008 hit song "Give It 2 Me" and Psy's "Gangnam Style," Madonna surprised the crowd by introducing the Korean singer, who promptly appeared in the center of the stage to a wild reception from the audience.