Fister threw six scoreless innings for Double-A Harrisburg, allowing only two hits, while walking none and striking out six.
"The numbers were good, 70 pitches through six innings," Williams said before Saturday's game against the Brewers. "He was efficient, threw all of his pitches for strikes.
"He's back in D.C. today, doing some throwing. We'll get him through a bullpen [session] and see where he's at."
Fister could be slotted back into the rotation if he's able to get through the bullpen session without any soreness or pain, according to Williams, who liked what the reports say about Fister's rehab outing.
"[Friday] night, he threw it where he wanted, which is a good sign," Williams said. "All indications are that he's feeling good."
Fister's forearm injury had affected his location of pitches and velocity. The right-hander is 2-2 with a 4.31 ERA in seven starts, almost a run higher than his career average of 3.38.
The Nats, whose rotation has struggled recently, are looking forward to get Fister back. Entering Saturday, the club had dropped eight of their past 10 games and surrendered an average of 5.87 runs per game.