TWENTY-ONE years old, 5-foot-1 and female are hardly the stats of a conventional heavyweight. But England’s Lady Sovereign proved herself a rapper of considerable talent and heft when she faced down three of music most formidable figures – R&B superstar Usher; Island/Def Jam chairman L.A. Reid and the label’s CEO, Jay-Z.

Lady Sovereign, known as “Sov” to her friends, interrupts a cherished game of “Guitar Hero” – “you ruined it for me” she chides The Post when we call – to describe her trial by fire at the hand of the three superstars, who asked her to freestyle without warning.

“I was like, ‘Oh, f – – – in’ hell,'” says Sovereign, who opens for Gwen Stefani tonight at Jones Beach, in her clipped British patter.

That the audition led to a deal proved a boon for the hip-hop honchos. Lady Sovereign’s 2006 full-length debut, “Public Warning,” earned her widespread critical acclaim, and the single “Love Me or Hate Me” has been both No. 1 on Billboard’s dance chart and the first-ever video by a Brit to top the chart on MTV’s “TRL.”

Sovereign, born Louise Harman, built her following in grass-roots fashion, promoting her first recordings, which she now calls “gibberish,” on pre-MySpace Internet sites.

“I don’t get the credit I deserve for the Internet thing,” says Sovereign, who currently has more than 220,000 friends on MySpace. “I might not have used MySpace, because it wasn’t out back then, but I grabbed people’s attention.”

While establishing her following, the side-ponytailed spitfire developed detractors, as well. Some assailed her for being a white girl in hip-hop. Others trashed the housing-project native for supposedly being “chav,” derogatory, classist British slang referring to those with hip-hop dress and crude behavior.

“At first it was upsetting, but as time went on I found it funny,” she says. “In the U.K., they single out anyone who’s young and fresh and hasn’t gone through stage school. But America gives me confidence.”