SEATTLE -- Brad Miller thought the ball would stay inside the foul pole. His manager wasn't so sure.
Miller came up with the bases loaded and no outs in the fifth inning Saturday and drove Jarrod Parker's 2-1 pitch high down the right-field line. The ball stayed fair for the rookie's first grand slam.
"It kept curving, but I was hoping," Miller said. "That felt good."
The slam was the second of two home runs Miller hit against Oakland, and it gave Seattle a six-run lead. The Athletics mounted a comeback but the Mariners held on for a 7-5 win.
"I wasn't sure if that ball was going to stay fair, but I'm glad for us it did," Seattle manager Eric Wedge said. "It was obviously big for us, especially the way they came back."
Boston clinched home-field advantage throughout the postseason with Oakland's loss. The AL West champion A's will play their playoff opener at home next Friday against AL Central champion Detroit, and AL East winner Boston will start at Fenway Park against the team emerging from the wild-card playoff: Cleveland, Tampa Bay or Texas.
Oakland's comeback try was hurt by many missed chances. The A's failed to score after loading the bases with two outs in the sixth and eighth innings, and they were 2 for 10 with runners in scoring position.
However, it was clear after the game the Athletics were already turning their thoughts toward the upcoming series against the Tigers, who beat Oakland 3-2 in last year's division series.
"It was a good matchup last year. We've had some spirited games with them this year, so it should be an exciting series," A's manager Bob Melvin said. "Two pretty well-matched teams, a little different in style. You look at the numbers overall, their pitching numbers and their hitting numbers, and ours and what we do and it's a little different, but should be pretty evenly matched."
Parker (12-8) gave up seven runs and seven hits -- including three homers -- in 4 1/3 innings. It was the second time in three starts he allowed seven or more runs without making it through the fifth inning.
"I think he was just a little off on the baseball today, but the stuff was still good," Melvin said. "I'm not worried about that."
Justin Smoak's career-best 20th homer, a two-run drive, put Seattle ahead in the second. Smoak became the fourth Mariner this season with 20 homers, the first Mariners quartet since 2007.
Miller hit his first homer of the game with a solo shot in the third for a 3-1 lead. Two innings later, the Mariners had three straight singles to lead off the inning and Miller extended the lead with his grand slam.
It was the third time this season Miller has hit two homers in a game, with the bulk of his eight homers coming in pairs. The grand slam was Seattle's seventh this year, second in the majors behind Boston's nine.
Brandon Maurer (5-8) gave up two runs and six hits in 5 1/3 innings. He had been 0-1 in three starts and three relief appearances since beating the A's on Aug. 20.
"I got to throw everything together this last one, work on mound presence and really go out there and trust my stuff," Maurer said.
Seattle manager Eric Wedge said Saturday his uncertain contract status wasn't the reason he decided not to return next season. "If they'd offered me a 5-year contract, I'm not coming back here," he said. Instead, Wedge said the reason behind his departure was a difference of opinion between him and the Mariners' front office: CEO Howard Lincoln, president Chuck Armstrong and GM Jack Zduriencik. "My vision of the future and theirs are just different," Wedge said. ... Oakland OF Yoenis Cespedes was out of the lineup a day after leaving in the fourth inning with shoulder stiffness. Manager Bob Melvin said Cespedes was day to day. ... Danny Farquhar pitched a perfect ninth for his 16th save in 20 chances. ... Coco Crisp hit a sacrifice fly in the third, and Alberto Callaspo had a run-scoring single in the sixth for Oakland.