The underwear were discovered rolled up inside the white jeans that had been removed from her and dumped in a bin at Wolverhampton's West Park where she was brutally bludgeoned to death in an attack that started in the wooden pavilion know as the 'black house,' the city's Crown Court was told.
Forensic scientist Lorna Fergus said in evidence that the odds of the DNA belonging to anybody other than Saidas Valentinas had been worked out by a computer to be one in a billion.
She added: "These were only trace levels of semen. It could have come from recent or historical sexual activity or innocent transfer.
"He and Viktorija shared a house as well as a laundry basket and washing machine."
In these circumstances the DNA could have been innocently transferred from an item of his clothing to the underwear but Mrs Fergus admitted she was unable to to say definitively how it came to be there.
The 16-year-old defendant has now admitted having sex with the teenage victim on the night she died at the park where she had arranged to meet a Facebook friend late at night.
The court was told the chances of the DNA found not being that of the accused were also one billion to one.
Mrs Fergus said she was unable to assist the jury in either deciding if the sex was consensual or whether it took place before or after Viktorija had died.
Similarly the forensic scientist was not in a position to comment on whether the defendant participated in the death.
Viktorija, who lived in Mostyn Street, Whitmore Reans, was semi naked when her body was found draped over a bench at West Park early on April 12 but the clothes she still wore were damaged in a way that could have been caused by her being dragged face down over the ground from the pavilion by the ankles or feet with her arms out stretched, maintained Mrs Fergus.
Pink ankle boots believed to have been worn by the deceased were recovered from the park's lake.
A low level mixed DNA profile was found in them but was nothing to do with the defendant, the jury was told.
The 16-year-old, who cannot be named because of his age, denies murder, rape and having sex with a dead body.
The trial continues tomorrow.