Karima el-Mahroug was ordered by the court to appear Monday to testify after failing to show on two previous dates, reportedly because she was in Mexico on vacation. She has been called as a defence witness.
Berlusconi is accused of paying for sex with el-Mahroug, better known as Ruby, when she was 17, and then trying to cover it up. Both deny sexual contact.
El-Mahroug is the last witness due to testify, meaning that a verdict could come before the Feb. 24-25 elections in which Berlusconi is heading a centre-right coalition.
Berlusconi's defence is expected to request a suspension in the trial for Italy's electoral campaign.