DETROIT -- It's been a long season for Justin Upton, one he hopes has begun to shorten up.

Upton signed a six-year contract with the Detroit Tigers totaling $132.75 million, a deal containing an opt-out clause after the second season.

Maybe feeling the pressure of having to live up to that deal, Upton got off to a slow start that quickly turned into a nightmare. He struck out early, struck out late and struck out in between.

The moment that suggested things might have turned for the Tigers left fielder could have come Monday night when he hit two home runs, the second a walk-off homer leading off the 12th inning to give Detroit an 8-7 victory over the Seattle Mariners.

"I don't think he'd admit it," Detroit manager Brad Ausmus said, "but I think he was pressing a little bit. I think the home run to center released a little bit. I think the walk-off, he should feel very good about himself."

Detroit snapped a three-game losing streak with the win while Seattle lost its eighth in its last 11 and 16th out of 24.

"It was a crazy game," Seattle skipper Scott Servais said. "It was kind of the battle of the bullpens.

"We had a couple big hits. We scored seven, and we had opportunities but we couldn't add on. We had our chances. We had plenty of men on base."

The Mariners stranded 12, getting a runner to third with one out in the 12th but leaving him there as Anibal Sanchez (4-7) got a groundout to drawn-in shortstop Jose Iglesias and a lineout to third.

Upton entered the game hitting .234 and had two strikeouts plus a walk before cranking his seventh home run of the season over the 420-foot marker in dead center. His game-winning eighth homer went to left-center. He hit a 2-2 offering from Vidal Nuno (0-1), who was coming out for his third inning of work.

"Nuno has been really good about taking the ball and going out there," Servais said. "He's a grinder."

"It's a big relief," Upton said. "We went 13 innings (Sunday) and almost that (Monday).

"It was great to win a ball game for this team. There's been a lot where I could have won the game but I didn't. My teammates have been picking me up all year."

Upton's two-run home run to dead center field with one out in the seventh inning gave Detroit a 7-6 lead, but Seattle tied it in the eighth on a two-out double by Ketel Marte followed by an RBI single by Leonys Martin.

Mike Pelfrey came into the game having allowed 90 hits and 30 walks in 71 1/3 innings, and he gave up 12 hits plus a walk in his five innings of work Monday. Nine hits were singles, two home runs.

Nathan Karns also pitched five innings, allowing seven hits and striking out six while walking three.

A 2-0 Detroit lead in the first achieved on a mammoth home run to center by Miguel Cabrera was wiped out by four Seattle runs in the second.

Kyle Seager led off with his 14th home run, Marte had an RBI single and Seth Smith lined a two-run single to right.

Cameron Maybin walked with one out in the first against Karns and Cabrera hit his 15th home run, which was logged at 454 feet. It landed atop the concrete catwalk adjacent to the center-field background and skipped into the street beyond.

The Tigers loaded the bases in the second on a single, double and hit batter with nobody out but only managed one run, when Maybin grounded into a double play to cut their deficit to 4-3.

Maybin's two-run triple in the fourth gave Detroit back the lead 5-4, but Adam Lind hit a two-run home run in the fifth, his 10th, to put Seattle back on top 6-5.

NOTES: Journeyman RHP Dustin Molleken, 31, has been brought up from Triple-A Toledo by Detroit to help an overworked bullpen. He has pitched for 11 minor-league teams over 13 seasons without throwing a pitch in the majors. ... Seattle RHP Taijuan Walker reportedly has returned to Seattle to have an MRI on the sore tendon in his right foot. He pitched five shutout innings Sunday but was removed due to the injury. ... Detroit IF Casey McGehee has been designated for assignment after making one pinch-hit appearance on Saturday. He had been called up earlier on that day. ... RHP Zach Lee, acquired by Seattle on Sunday night for SS Chris Taylor, will be slotted at Triple-A Tacoma by the Mariners.