Namibia opened the second half with new resolve, but blew a scoring chance in the 43rd minute. Wales went on an immediate counterattack, only to ruin their own opportunity at the other end. However the momentum gained would carry Wales through and after some resistance, the points began to flow as the Namibians began to tire.
Williams delivered that desired bonus point in the 46th minute after powering through five defenders and finishing with a swan dive to score his second try of three, putting a dampener on the revved-up Namibians.
121kg prop Gethin Jenkins scored the try of the night with a 45m run, charging through tacklers and selling a quick dummy on his way to the line and a five-pointer that appeared to break Namibia's resolve. A minute later, a Welsh pass was intercepted by standout performer Nico Esterhuyse. The 205cm lock bolted upfield and drew in two defenders before offloading to lock Heinz Koll, who pinned his ears back and beat a gang of Welsh chasers to score his second try of the tournament.
First-five Theuns Kotze added the extras and in doing so set a new international points-scoring record for Namibia.
That was enough for Wales, who quickly turned up the pressure and ran in three tries in five minutes in what was a merciless display of attacking rugby.
Namibian flanker Tinus Du Plessis was named man of the match but any member of his team's forward pack could have earned the honour, including skipper and opposite flanker Jacques Burger.
After the match, Burger thanked fans for their support and wished the All Blacks well in their pursuit of the Webb Ellis Cup.
Wales are second-placed in Pool D with two wins and a game still to play against Fiji.
WALES 81 (G Jenkins, A Jones, T Faletau, A Brew, S Williams (3), J Davies, L Byrne, L Williams, G North (2) tries; S Jones (6), R Priestland (3) conversions; S Jones penalty) def. NAMIBIA 7 (H Koll try; T Kotze conversion)
Halftime: Wales 22 - Namibia 0