Strauss-Kahn has said that he never met brothel owner Dodo, real name Dominique Alderweireld, who is charged with providing women for paid services in France.
The disgraced former International Monetary Fund chief, widely known in France as DSK, is due to take the stand next week.
The trial is the latest in a series of cases offering a peek behind the bedroom door of a man once tipped as a potential challenger to former French president Nicolas Sarkozy.
France was stunned when it saw Strauss-Kahn paraded handcuffed in front of the world's cameras after a New York hotel maid accused him of sexual assault in May 2011 — a case that was eventually settled in a civil suit.
In the current case, Strauss-Kahn, 65, admits to being a "libertine" who took part in orgies, but denies knowing that the women at the parties were prostitutes.
The economist nonetheless finds himself facing 10 years in prison and a fine of up to €1.5 million ($1.7 million) for "aggravated pimping in an organised group", with investigating judges arguing he played a role in initiating the sex parties and organising the presence of the prostitutes.
Frenchman Alderweireld is alleged to have employed the prostitutes that attended the sex parties with Strauss-Kahn.
One of Alderweireld's sex clubs in Belgium was called "Dodo Sex Klub" (DSK), but he denied being a professional pimp.
He assured the court that the girls in his establishments acted of their own free will, notably when it came to travelling to Lille, northen France, for paid encounters.
"I can't have anything but independent (girls), because if there is some kind of subordination, then under Belgium law I would be a pimp," he said.
The women may have had money problems, which led them to take up the offers, he added, but "I'm not the one in charge, I'm not the boss. I am not the director of the IMF!" he told reporters outside the court.
His companion, Beatrice Legrain, 41, who manages one of his clubs, told the court that she was an occasional prostitute because "I like it."
But ex-prostitute Jade, who worked at one of Dodo's club in Belgium, cast doubt on his suggestion that he wasn't a professional pimp when she gave her testimony in court.
"I never met any girl with independent status, I was the only one," she said.