A Sixers official told ESPN the request came on Tuesday and the organization granted it.
The Suns plan to compete with the Los Angeles Lakers in pursuit of Williams, league sources said. The Suns see Williams as a coach who can develop a young roster and bring discipline to a fractured Suns culture.
After firing coach Igor Kokoskov on Monday night, the Suns will be hiring their fifth coach in five years.
Williams is working to set up a second interview that includes Lakers ownership in Philadelphia soon, league sources said. Williams and Tyronn Lue are prominent candidates in the Lakers' coaching search.
Suns general manager James Jones played under Williams in his time as a Portland Trail Blazers assistant coach, and new senior vice president of basketball operations Jeff Bower hired Williams as head coach of the New Orleans Pelicans in 2010.