SEATTLE -- As strong as the top of the Houston Astros' lineup is, it was the back end that did the most damage to the Seattle Mariners the past two nights.

Or just one guy in the back end of the lineup.

Cameron Maybin hit a tiebreaking, two-run homer off Seattle closer Edwin Diaz in the ninth inning Wednesday, and the Astros defeated the Mariners 5-3 to complete a three-game sweep.

It was the second consecutive night Maybin hit a go-ahead homer to beat Seattle after Houston acquired him Aug. 31.

"We knew everybody would like him," Astros manager A.J. Hinch said. "He's obviously performing at a high level, earning more at-bats. Big swing tonight."

The Astros (86-53) ran their winning streak to seven games. Houston increased its lead over the second-place Los Angeles Angels in the American League West to 14 1/2 and cut its magic number to win the division to nine.

The Mariners (69-71) lost their third straight and dropped 3 1/2 games back of the Minnesota Twins in the race for the second AL wild card.

"Disappointing series, no doubt," Mariners manager Scott Servais said. "I thought we were right in every game, we just didn't get big hits late, and that's what it takes.

Diaz (3-6) gave up a leadoff double to Carlos Beltran in the ninth. Two batters later, Maybin took Diaz the other way, homering off the right field foul pole.

Maybin joked that guessing what was coming from Diaz wasn't the hardest part. Not against the heat Diaz throws.

"He throws 100 (mph), so I think it's pretty simple," Maybin said. "Look for one pitch when he throws that hard."

Chris Devenski (8-3) earned the win, pitching 1 1/3 scoreless innings to close out the contest.

Early on, the game had the makings of a pitchers' duel between Houston's Lance McCullers Jr. and Seattle's Andrew Moore, two pitchers who had just returned to their rotations this week -- McCullers from the disabled list, Moore from the bullpen.

McCullers was dominant early, retiring the first 11 Mariners. Except for a two-run double by Marwin Gonzalez in the fourth, Moore did a good job of dodging trouble.

Seattle's Kyle Seager hit a solo homer in the fifth.

The wheels came off for McCullers in the sixth, when he surrendered a leadoff triple to Jean Segura before Robinson Cano's flare to left equaled the score at 2-2. Mitch Haniger's two-out single brought home Cruz and gave Seattle a 3-2 lead.

McCullers went 5 1/3 innings, allowing four hits and three runs, before giving way to Francisco Liriano after coughing up consecutive hits to Cano and Nelson Cruz.

Moore went six innings, allowing two runs on three hits and two walks while striking out three. Servais said Moore's start went beyond his hopes.

However, the bullpen couldn't hold the lead. George Springer evened the score at 3-3 with an RBI single in the seventh off Nick Vincent.

Once again it was the seventh inning that bit the Mariners. It was the same juncture when Houston produced a decisive four-run rally in the series opener, then Maybin's go-ahead shot Tuesday.

Now, Springer's tying single on a lazy liner to center.

"For whatever reason, the seventh inning has been our Achilles' heel in this series," Servais said. "And it got us."

Meanwhile, the Astros look to be finding their stride at the right time of year, with the division title in sight.

"We're pitching, we're hitting, we're playing with some enthusiasm," Hinch said. "Pretty good run."

NOTES: Houston 2B Jose Altuve took a routine day off, and Marwin Gonzalez took his spot in the field. Astros manager A.J. Hinch said Altuve would be back in the lineup Friday in Oakland. ... Astros OF Jake Marisnick (groin) returned to action after missing five games, pinch-running for Carlos Beltran in the ninth. ... Houston RHP James Hoyt was recalled from Triple-A Fresno and slotted into the bullpen. ... Seattle RHP Felix Hernandez (shoulder) and LHP James Paxton (pec) will pitch in a simulated game on Friday. Each is expected to throw 30-35 pitches. ... Mariners RHP Seth Frankoff, claimed off waivers from the Chicago Cubs on Tuesday, was optioned to Class A Modesto.