Brittney Griner had 13 points, eight rebounds and five assists for the Mercury, while Diana Taurasi and Candice Dupree chipped in 11 and 10 points, respectively, to help Phoenix win its eighth straight game.
Angel Robinson led Storm scorers with 10 points off the bench. Crystal Langhorne netted nine points and 10 rebounds for Seattle, which has dropped three of its last four games.
Phoenix used a 19-0 surge in the second quarter to pull away from Seattle and outscored the Storm 21-4 for the period. The teams entered the stanza tied at 16-16.