She's a hissing, claws-out socialite on the hit Bravo reality show "Real Housewives of New York City," but Kelly Bensimon turned into a butt-kicking boyfriend beater in her posh lower-Manhattan coop last week, court records claim.

The 6-foot-tall, 40-year-old former model, horse fancier and one-time marathon runner got into a fight last week with her boyfriend, 30-year-old Nick Stefanov, and clocked him, giving him a black eye and opening a blood-gushing gash on his left cheek, according to the records.

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Stefanov fled her apartment on Centre Street after the Tuesday-morning beating and reported the mother of two to cops at the 5th Precinct station.

A source said Bensimon at the time was sporting "a fat lip."

Bensimon surrendered two days later and was charged with misdemeanor third-degree assault. She's due in court March 31.

"We got engaged a couple weeks ago," Stefanov told one friend. "We got into an argument that escalated and escalated. She hauled off and started swinging. She got a lucky punch on my cheekbone and just split it right open. I went down to the police station covered in blood."

RELATED: Go to NYPost.com for the full report.