Sarah was 18 when she posed next to the BGT judges for a photo in 2016.
She said he later contacted her on Instagram and booked her and an 18-year-old friend a spa day at the Bulgari hotel in Knightsbridge, London, and invited them to his house, Supernova Heights.
Sarah claims both women became drunk, but Sarah said when she got up to go to the bathroom, Walliams followed her.
She said he then put his hands around her waist and moved in to kiss her.
“I had a boyfriend at the time, so I was just like, ‘Absolutely not.’” When she looked at him, she said she remembers thinking: “You’re the same age as my mum.”
Walliams’ lawyers said he “refutes the suggestion he got (Sarah) drunk to take advantage of her”.
Although he recalls they drank wine together, they said, he does not recall anyone being drunk.
They added Walliams did not force himself on Sarah and there was no sexual contact with her.
Both women were friendly the next day and did not raise any issue with Walliams, they said.
The lawyers denied he targeted the teenage women and used his celebrity status to manipulate them to have sex with him.
Sarah later received a “threatening” letter from Jenny Afia from the law firm Schillings.
Afia is best known now for representing the Duchess of Sussex.
It warned her she would be committing a “civil offence” if she disclosed any “private information” concerning Walliams.
A spokesman for Schillings said they had been instructed to write the letter, as they had understood the woman was going to “sell her story”, which Walliams believed was “an untruthful account”.