"I am sorry from my heart," she said in a statement read by her lawyer at the opening of her trial in the German city of Darmstadt on Monday.
"No way did I want my partner to be infected," she told the administrative court.
In her statement, Benaissa told the court she became a cocaine addict at 14 and that during her pregnancy at 16, she found out she was HIV positive, according to the news agency ddp.
Five more sessions are scheduled in her trial. It is not clear if Benaissa will give any further testimony.
The man who claims the singer infected him says they had a three-month relationship in early 2004, and that he got tested for HIV after the singer's aunt asked him in 2007 whether he knew Benaissa was HIV-positive.
Al Jazeera's Lawrence Lee reports on German pop star accused of infecting man with HIV
In 2000 Benaissa won a television talent show Popstars and joined No Angels with four other young women but hid her illness from everyone for fear it would damage her career.
"I grew apart from everyone, even myself," she said in the statement about that period of her life.
No Angels sold more than 5 million albums before breaking up in 2003.
Along with three other members from the original band, Benaissa helped reform the group in 2007 but performed with disastrous results in the 2008 Eurovision song contest.
No Angels were heading into a concert in Frankfurt in April 2009, when Benaissa was taken into custody and kept for 10 days – a move a German AIDS awareness group has criticised as disproportionate.
The Deutsche AIDS-Hilfe group argued that the question of whether her partners also carried a share of the responsibility had been neglected.