The day after Donald J. Trump said accusations of inappropriate sexual contact with nine women were “totally false,” a new accuser came forward on Thursday and described an encounter in which she said Mr. Trump grabbed her arm and fondled her breast.

Karena Virginia, a yoga instructor and life coach from the New York region, said she was 27 at the time and was waiting for a car to take her home outside the United States Open tennis tournament in 1998.

She overheard Mr. Trump, who she said she had never met, speaking to a group of men about her legs.

Mr. Trump approached and grabbed her right arm, Ms. Virginia said. “Then his hand touched the right inside of my breast,” she added.

She flinched, she recalled. Mr. Trump said, “Don’t you know who I am?”

After her car pulled up, “my shock turned to shame,” Ms. Virginia said. “I felt ashamed that I was wearing a short dress and heels.”