Her testimony was heard on the first day of Alieu Kosiah's appeal to overturn 22 convictions for war crimes.
Kosiah was a rebel commander fighting against former President Charles Taylor's army in the 1990s.
In 2021 he was sentenced to 20 years in prison in Switzerland's first ever war crimes trial.
Swiss prosecutors are arguing that he also committed crimes against humanity, which could increase his 20-year sentence if his conviction is upheld and he is found guilty on these counts.
The public left the courtroom to allow the victim, who said she was raped by Kosiah at the age of 14, to continue her testimony.
"If I didn't go, he would have killed me. He forced me four times," she told the court.
Kosiah told the court he had never met the woman. His lawyer says he was not present when the war crimes were allegedly committed.
The rebel commander was arrested in Switzerland, where he had been living, in 2014.
Swiss law allows for prosecution for serious crimes committed anywhere under the principle of universal jurisdiction.
Thousands of people were mutilated and raped during Liberia's back-to-back wars between 1989 and 2003 in clashes that involved drugged fighters and child soldiers conscripted by warlords.
But unlike neighboring Sierra Leone, which had its own civil war at the same time and later held war crimes trials, no prosecutions have taken place in Liberia.
The victims testifying in Switzerland have asked for anonymity for fear of reprisals.
Some former warlords still hold major positions in the country.
The court will later hear a man who alleged he witnessed Kosiah eat slices of a man's heart and another who accuses him of stabbing him and ordering killings.
