The win lowered Detroit's magic number to clinch the American League Central to four over the Cleveland Indians.
Seattle used a three-run home run by Dustin Ackley in the fifth inning to grab a 4-3 lead.
But Detroit's Torii Hunter led off the bottom of the seventh with a double and he took third on a fly out by Miguel Cabrera. Fielder followed with a single to right off Charlie Furbush (2-6) that tied the score. Victor Martinez hit a drive off the left-field wall for a double and Fielder motored in from first with the tie-breaking run.
Raul Ibanez singled in a run to complete a two-out rally and give the Mariners 1-0 lead in the first. Detroit answered in the bottom of the inning with a home run from Hunter and the first RBI double by Martinez on the day. Fielder also scored on that hit, making the score 5-4.