Hernandez (9-4) ran his unbeaten streak to eight starts, limiting the Red Sox to two runs on six hits. The Mariners ace walked two and struck out six.
Saunders drew a bases-loaded walk in the fourth, giving Seattle a 2-0 lead, and later added a pair of run-scoring doubles as Seattle banged out 15 hits in the opener of a four-game series.
Boston starter Jon Lester (8-5) was tagged with the loss, surrendering five runs on nine hits over five full frames.
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