"I've never done it the way I'm gonna do it tonight," Aguilera said prior to the standout performance, which featured a number of tempos, ranges and instrumental accompaniments.
McClellan said she was "honored" that "Christina ... shared this song with me as a duet ... I couldn't have asked for anything better -- I feel blessed."
Aguilera called McClellan a "once-in-a-lifetime" singer with unbelievable range.
Find out who wins The Voice when it hold it’s big finale Wednesday at 8/7c on NBC.