Gellar played the Good Morning, Vietnam star's daughter in Tv comedy The Crazy Ones and was so upset by the news of his passing on Monday (11Aug14), she simply posted photos of herself with Williams on her social media accounts.
She has since updated her Twitter.com page with the note, "I just want to say thank you for all the kind words and tributes. The outpouring of love is spectacular and greatly appreciated."
And the actress tells People.com, "My life is a better place because I knew Robin Williams. To my children he was Uncle Robin, to everyone he worked with, he was the best boss anyone had ever known, and to me he was not just an inspiration but he was the father I had always dreamed of having.
"There are not enough adjectives to describe the light he was, to anyone that ever had the pleasure to meet him. I will miss him everyday, but I know the memory of him will live on. And to his family, I thank them for letting us know him and seeing the joy they brought him. Us crazy ones love you."
She then quoted the beginning of the Serenity Prayer, which is used as a meditation for recovering addicts, like Williams, who checked in to rehab earlier this summer (14) to focus on his sobriety.
She said, "God, grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference."
Williams committed suicide at his home in Marin County, California on Monday, aged 63.