Rovers rode their luck in a one-sided first half and came up with a precious second-half goal from the former Glovers midfielder.
After three successive wins a confident Yeovil went looking for goals but missed a host of chances in the first half with Oli Johnson, Dean Bowditch and Andrew Welsh all going close.
They continued where they left off at the start of the second half and a header from Craig Alcock came off the underside of the bar.
Rovers brought on Williams after an hour and he tapped in the vital goal from close range in the 76th minute after a dazzling dribble by skipper Will Hoskins.
Source: PA
Source: PA