HOUSTON -- With his bullpen stretched thin following a long but successful weekend in Chicago, Seattle Mariners manager Scott Servais was looking for precisely the type of all-around effort his club produced on Monday night against the frontrunners in the American League West.

Kyle Seager and Danny Valencia clubbed back-to-back home runs in the 10th inning and the Seattle Mariners completed a wild 9-7 win over the Houston Astros at Minute Maid Park.

Seager, who hit his 13th homer, and Valencia, who smacked his 11th, completed a four-homer night for Seattle (47-47) by taking Astros left-hander Tony Sipp (0-1) deep. That power surge made a winner of Mariners right-hander Yovani Gallardo, who benefited from three strong defensive plays by shortstop Jean Segura as Houston loaded the bases in the ninth.

The last of those plays was a backhanded stop of an Alex Bregman grounder that led to a force at second that got Brian McCann with the bases full.

"I knew the only chance I had was the force at second," Segura said of his final play in the ninth. "It was just 'go get it.' It was the last out and if I don't get it or make an error, they win the game. I tried to make the play as quick as I can."

Sipp was the seventh pitcher of the night for the Astros (62-31), and the third reliever to yield a home run. The Mariners went seven relievers deep after the Astros chased Seattle starter Ariel Miranda with a four-run sixth inning.

Gallardo (4-7) escaped the ninth and handed the ball and the lead to Edwin Diaz, who breezed en route to his 17th save on the season.

"We had pitchers make big, big pitches tonight, (and) Gallardo getting out of that jam he was in," Seager said. "It was a really good all-around win for us."

Momentum swung back and forth once the Mariners chased Astros starter Lance McCullers, with Houston posting a four-run, two-out rally in the sixth to erase a 5-2 deficit. Seven consecutive batters reached base including right fielder George Springer, whose bases-loaded walk off Tony Zych forced in McCann as the go-ahead run.

But the Mariners responded with a pair of solo blasts, first from Nelson Cruz off right-hander Chris Devenski in the seventh inning to tie the game at 6-6. Mike Zunino added his 13th homer in the eighth, pushing the Mariners ahead with his shot to left-center off Luke Gregerson.

"Clearly, we struggled with making pitches the whole night, starting with Lance," Astros manager A.J. Hinch said. "It is tough to lose that game. We had plenty of opportunities. We kept battling, they kept battling and they win."

The Astros matched that tally in their half of the eighth, with Springer plating Norichika Aoki with a sacrifice fly after the first three batters reached base to open the inning.

For a brief spell, McCullers appeared to right himself after his first-inning throwing error allowed Segura to score from second with one out.

McCullers retired nine consecutive batters before the Mariners forged a rally in the fourth inning and knocked out the right-hander in the fifth.

After McCullers issued the second of his two walks in the fifth, he was pulled. Seager hit his second infield single, this one a run-scoring hit, before McCullers departed after allowing five runs (four earned) on six hits and three walks over 4 2/3 innings.

Over his last three starts, McCullers has allowed 14 runs (12 earned) on 21 hits in 14 1/3 innings.

"I didn't do a very good job of controlling some big at-bats, especially there in the fourth," McCullers said. "I'm not going to look too much into it. Our team is in a great spot. The offense did a great job of fighting back and getting us back into it tonight."

NOTES: Astros RHP Collin McHugh made his last rehab start with Double-A Corpus Christi on Sunday night, allowing one unearned run while tossing 69 pitches over six innings. McHugh ran his pitch count past 90 with additional work in the bullpen. He will throw a bullpen session on Tuesday and is tentatively scheduled to rejoin the bullpen this weekend in Baltimore. McHugh has missed the entire season with a posterior impingement in his elbow. ... Mariners OF Mitch Haniger did some light work in the batting cage and took soft toss but missed a second consecutive game with a jammed finger sustained on Saturday. The club is hopeful that he can return to action on Tuesday. ... Mariners RHP Sam Gaviglio will be recalled from Triple-A Tacoma and will start on Tuesday night. Gaviglio was optioned to the Rainiers on July 7 and suffered a 5-1 loss to the Fresno Grizzlies on July 13, allowing four runs on three hits and two walks over five innings.