Christina Aguilera NAILS Super Bowl Rehearsal

Christina Aguilera sang an UNBELIEVABLE rendition of the "Star-Spangled Banner" during Super Bowl rehearsals -- one of the best ever -- and she was so confident in her performance, she didn't even finish the song!!!

TMZ obtained footage of the National Anthem rehearsals -- shot Saturday in Cowboys Stadium.

Christina not only gets all of the lyrics correct -- but the delivery was Whitney Houston-esque.

So much for the maxim practice makes perfect ...