The American sisters claimed that they, along with their father Richard, had been the victims of racial slurs during the tournament in California in 2001.
"I don't think you ever say it's a lost cause [to see them return]," tournament director Steve Simon told BBC Radio Five Live. "We have had communications with them over the years, many times, and we actually get along very well with them.
"They unfortunately had, in their minds, a bad experience here and have decided not to come back, but they know that they are welcome here and we get along famously. But they've got their reasons for not wanting to do it, and that's the way it is."
Meanwhile, US Open champion Andy Murray could move into the world number two spot if he reaches the final of the competition and Roger Federer fails to progress to the semi-finals.