"Fighter" is a pop song that features elements of rock music. Sal Cinquemani from Slant Magazine said that "Fighter" is a hybrid of R&B and rock. Meanwhile, a reviewer of "Billboard" described the song as arena rock-influenced, and AOL Music's Nana-Adwoa Ofori noted the elements of rock and roll. However, Nick Levin from Digital Spy detailed "Fighter" as an arena rock piece. The song was inspired by band Guns N' Roses' song "November Rain" (1992). Written in the key of E minor, "Fighter" has a moderate tempo of 96 beats per minute. Brian Hiatt from "Entertainment Weekly" and Chuck Taylor for "Billboard" likened the track's musical style to that of Janet Jackson's "Black Cat". Aguilera's vocals on the track span from the low-note of G3 to the high-note of G5; Taylor labelled Aguilera's vocals "tornado of a vocal."