Saunders (1-1) scattered six hits over 6 1/3 scoreless innings, issuing only one walk while striking out five.
Carter Capps and Charlie Furbush combined for 1 2/3 frames of shutout relief and Tom Wilhelmsen notched his third save with a 1-2-3 ninth as the Mariners claimed the first of a three-game set.
Kendrys Morales supplied a pair of run-scoring hits for Seattle.
Phil Humber (0-1) was touched for three runs on five hits in a six-inning start, allowing one walk while striking out two for the Astros, who fell for the sixth time in seven games to start the season.
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