Rendon has yet to play in the Majors this season because of a sprained MCL and strained oblique.
Williams also announced that right-hander Doug Fister (flexor strain) and left-hander Sammy Solis (inflammation in shoulder) threw on flat ground on Friday.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.