Earl Watson scored 18 points and had seven assists for Seattle, which played without injured leading scorer Kevin Durant. Nick Collison finished with 15 points and 11 rebounds, and Jeff Green chipped in 16 points.
The Sixers saw their double-digit lead in the second half slip to just three following a Delonte West three-pointer with 2:50 left. But Seattle missed its next two shots and Williams was fouled twice. He made all four shots to give the Sixers a 93-86 lead with just under two minutes left that held up the rest of the way.
Seattle led 23-21 at the end of the first quarter, but Philadelphia ran off eight straight points to lead 46-34 at halftime and was up 76-32 after three periods.