Brown was seen speaking to Lewis at a pre-Grammy awards party in Los Angeles hours before the attack The Daily Mirror claims.
A source told the newspaper: "Rihanna saw Chris flirting. He and Leona were laughing and Rihanna saw his hand on the small of her back.
"It was totally innocent but it set Rihanna off."
The couple later left the party at LA’s Beverly Hills Hilton and drove off in Brown’s Lamborghini.
The unnamed source added: "Leona is very happy with her boyfriend Lou and it was totally innocent on her behalf. She wasn’t flirting."
American website TMZ.com has reported that the couple argued over a text message Brown received from a mystery woman.
Police sources quoted on the website said the Umbrella singer, 20, lost consciousness during the alleged attack in the early hours of Sunday morning, as Brown choked her and screamed: "I’m going to kill you!"
One source said that Rihanna came round to find that her 19-year-old boyfriend had fled.
Brown later turned himself in to police, was charged with making criminal threats and released on $50,000 (£34,000) bail, although more charges may follow.
The Barbadian singer received medical treatment at Cedars-Sinai Medical Center in Los Angeles for swelling and bruises after the argument with Brown in the early hours of Sunday morning.
In 2007, Brown claimed in an interview with American magazine Giant that his family had suffered domestic abuse at the hands of his stepfather.