She said she had 'toiled over every detail which could be manipulated'.
The case is due to last up to three days with judgment expected at a later date.
Jason Knauf, who was communications secretary to Meghan and her husband Prince Harry until March 2019, said in a statement to the Court of Appeal in London the duchess had told him that she had known it was possible that her father would release the letter.
London: The Duchess of Sussex understood that a letter at the centre of a privacy breach row with a British newspaper could be leaked by her father, a court was told Wednesday.
At a previous hearing, Meghan's legal team said Knauf had told them he did not write or help draft the letter.
He said Knauf's evidence cast doubt on the original ruling, adding that the case should have gone to a full trial.
It has not yet done so because of the appeal, in which lawyers for the publishers argue that the letter was written with the knowledge that it could be made public.