Glory Johnson added 15 points and Angel Goodrich had 14 for the Shock (4-13), who won without injured Skylar Diggins. The rookie point guard, who is averaging 8.6 points and 4.8 assists, injured her ankle against Minnesota on Saturday night.
Camille Little led the Storm with 11 points. Tina Thompson added 10 points, the only other player in double figures for Seattle (6-9).
The 6-foot-8 Liz Cambage helped the Shock to an early lead, scoring seven of her 13 points during a 10-2 run to open the game.
Williams made her first three 3-point attempts, scored 15 points after halftime and finished 9-for-15 from the field in 18 minutes.