HOUSTON -- Yordan Alvarez smashed a game-ending, three-run homer with two outs in the ninth inning off Robbie Ray, wrecking the Seattle Mariners' strategy of using a Cy Young Award winner in a rare relief role and vaulting the Houston Astros to an 8-7 victory Tuesday in their playoff opener.

Trailing all game after a poor start by Justin Verlander, the AL West champion Astros overtook rookie star Julio Rodriguez and the wild-card Mariners at the end to begin their best-of-five division series.

Houston was down 7-5 when rookie pinch-hitter David Hensley reached with one out in the ninth as Seattle closer Paul Sewald grazed his jersey with a pitch.

Sewald struck out Jose Altuve before Jeremy Pena laced a single to center field to chase Sewald.

Mariners manager Scott Servais then made the bold move to bring in Ray, who started Saturday at Toronto in the AL Wild Card Series, for a lefty vs. lefty matchup with Alvarez. Ray, who won the Cy Young last year with Toronto, had made only six relief appearances in his career and had never earned a pro save.

Alvarez, who hit 37 homers in the regular season, sent Ray's second pitch deep into the seats in right field to set off a wild celebration with his parents in the stands.

His blast was the second walk-off home run in postseason history by a team down to its final out, according to ESPN Stats & Information research. The other was Kirk Gibson's walk-off home run off Dennis Eckersley to lift the Los Angeles Dodgers to a Game 1 victory over the Oakland Athletics in the 1988 World Series.

The Mariners jumped on Verlander for six runs in just four innings to build a 6-2 lead early. Yuli Gurriel hit a solo homer in the Houston fourth before Eugenio Suarez's solo shot in the seventh extended Seattle's lead to 7-3.

A two-run homer by Alex Bregman off Andres Munoz cut the lead to 7-5 in the eighth inning to set up the dramatic finish.

The Associated Press contributed to this report.