Written by Dr. Luke, Claude Kelly and Benny Blanco, "Circus" is a metaphor for the public's perception of Spears' life.

"Circus" was well received by contemporary critics, with reviewers complimenting Spears' confident persona and praising the song's electronic production.

"Circus" was a commercial success, peaking inside the top-ten in Australia, Canada, Denmark, New Zealand, and Sweden, while reaching top-twenty positions in many European countries.

[1] On a global scale, "Circus" was one of the top ten best-selling songs of 2009 with 5.5 million digital copies sold that year across the world, according to the IFPI.

It was also the opening number of the Circus Starring Britney Spears (2009), where she was dressed in a ringmistress outfit, designed to represent a metamorphosis.

[8] The song opens with the sound of a drum and builds until Spears starts singing,[9] "There's only two types of people in the world/The ones that entertain and the ones that observe".

[4] Nick Levine of Digital Spy described it as "a rampaging monster of a song whose chorus kicks off with a Max Martin-style and Timbaland thump".

[14] Popjustice praised the production of the song, compared it to "Break the Ice" and added, "there's a lot of stopping and starting, including a built-in MTV-style dance breakdown segment.

"[9] Chris Williams of Billboard in its single review appreciated the song for surrounding Spears with "an electronic cyberpop landscape".

[15] In the album review, Ann Donahue of Billboard criticized the lyrics for rehashing the theme of fame and compared it to "Lucky" (2000) and "Piece of Me" (2007).

[16] Dave De Sylvia of Sputnikmusic said that while "Womanizer" sounded like a Blackout track, "Circus" is more reflective of the album's general mood.

"[18] BBC Music's Talia Kraines thought that the singer "loves the attention being shone directly on her as the 'ringleader' as she sings of adrenaline flowing through her veins when it's time to put on a show.

"[20] Jonny Mugwump of The Quietus said, "a paean to her performance adrenalin, ["Circus" is] a medium-paced, big power-popper topped with an orbit-esque guitar,"[21] and commented that the song's lyrics highlight Spears' uniqueness – "the fuck you attitude.

Despite everything, regardless of the personal destruction and undoubted hell she might still be haunted by, within the fictional space of the music, this woman couldn't give a damn.

[1] In Canada, the track debuted at number two based solely on digital downloads, being blocked from ascending to the top spot by Lady Gaga's "Poker Face".

[37] The music video opens with a frame of her hands picking up Curious, and also features a minimal but luxurious necklace and earrings by Bulgari.

In order of appearance, animals recorded in the video are doves flying out of a top hat, lions flanking Spears, and elephants doing their own dance steps.

One group dance scene takes place in the middle of a circus ring with Spears in a ringmaster jacket, sequined hot pants and a feathered top hat.

[37] Davil Balls of Digital Spy described the music video as being "cheeky, seductive and more than a little bit ridiculous" and said that Spears "delivers some killer dance routines".

Spears had previously received criticism from the organization when she used an albino python and a caged tiger during a performance of "I'm a Slave 4 U" at the 2001 MTV Video Music Awards.

[48] The show began with a video introduction featuring Perez Hilton as Queen Elizabeth I, welcoming the audience to the circus.

As the video ended, Spears started to descend from the ceiling on a suspended platform, wearing a cheetah headdress, a ringmaster jacket, black shorts, high-heeled boots and carrying a whip.

[48] She then ran into the center of the main stage, surrounded by jets of smoke, to enter a golden cage and perform "Piece of Me".

[49] People writer Chuck Arnold wrote that Spears, "descending from above in a red ringmaster-meets-dominatrix outfit" to sing the song, "showed that her body was tight, even if her dancing always wasn't.