Who did eminem sing with at the 53rd grammy awards?

i just got done watching this years grammy awards and at first rhianna was singing her part (or i thinnk its called love the way you lie part 2) of love the way you lie and then eminem comes and starts rapping then rhianna goes somewhere and this other girl comes out and i was wondering who this girl is. I remember when she sang she mainly said "i need a doctor"