Williams saved Scott Vernon's spot-kick seven minutes from time as Hibs progressed to the last eight of the competition with a wonder goal from Gary Deegan at Easter Road.
The Hibs stopper has been beaten only once from eight penalties in the current campaign, and Fenlon believes that is down to confidence.
"He has been fantastic all season," Fenlon said. "His penalty saves have been fantastic but his general all-round game is great.
"I think until we scored there was very little in the game. Once we scored it changed it a bit, but as normal we stood up well."
Hibernian's reward for their fifth-round victory is a trip to Kilmarnock and a third successive tie against Premier League opposition after seeing off the Dons and Hearts.
But Fenlon refused to discuss Hibernian's chances of ending their 111-year wait to win the Scottish Cup.
"I'm not getting into that. Since I've been here that's all I hear," he added. "There's a long way to go and we've got a really tricky tie away from home next."