The minor at the centre of the vice-ring case testified behind closed doors in a district court on Wednesday.
The 20-year-old woman was 17 at the time of her trysts with Swiss national Juerg Buergin in 2010 and 2011.
The 41-year-old former executive director of UBS is on trial for paying $600 for the minor's sexual services at Shangri-La Hotel on Sept 29, 2010, and $650 at Mandarin Marina Hotel on Jan 3, 2011.
The windows on the door of the court were plastered with white paper just before the minor was called to the stand during a short break after the pimp, Tang Boon Thiew, 39, had completed his evidence on the stand.
Tang, who used to run the online vice ring, is serving sentence for various offences of vice and abetting in commercial sex. He has appealed against his sentence of 58 months' jail and a $90,000 fine imposed earlier this year.
Defence lawyer Selva K. Naidu produced a document showing that his client Buergin had made bookings for the escorts' services over a period of about a year until October 2011. Some of the bookings did not go through but in all, he met 10 women, including the minor, in hotels such as St Regis, Grand Hyatt and Marina Mandarin. The other women were all above 18, the oldest being 43.