"Rihanna was served at my office by LA DA investigators," Etra said. "If the preliminary hearing indeed goes forward, she is now legally required to be there, she will be there and answer all questions truthfully."
Brown, 20, pleaded not guilty to two felony counts following the alleged beatdown of then-girlfriend Rihanna on Feb. 8.
Brown broke his silence last month when he told fans on a YouTube video from May 26 that all the "haters" should back off.
"All my real fans, I love y'all," Brown said in the video. "I ain't a monster."
Brown was denied his request yesterday to delay the June 22 hearing.
The case could end with a plea deal in which case the hearing could become unnecessary, according to People.
"Rihanna's testimony can make or break the case," LA criminal defense attorney Jeffery Rubenstein told People magazine. "Her willingness to testify and be truthful about what happened is a step closer to convicting Brown."