Tillman and the Orioles will look to continue their dominance of the last-place club when the teams begin a three-game series Monday night at Safeco Field.
Baltimore holds a 2 1/2-game lead over the Los Angeles Angels for the AL's second wild-card spot and trails the New York Yankees by one game for the East lead. It dropped two of three at Oakland over the weekend, but ended the series on a positive note with a 9-5 victory Sunday.
Matt Wieters hit two solo homers and scored three runs for the Orioles (82-64), who clinched their first winning record since finishing 98-64 to win the East in 1997.
''Every game right now is a must win,'' Wieters said. ''This is a tough place to win. To be able to get one, we'll take it, move on to Seattle and try to win a couple up there.''
Baltimore has won five straight meetings with Seattle after dropping the first matchup of the season, with Tillman (7-2, 3.39 ERA) picking up two victories against the team that drafted him in 2006.
Tillman's breakout 2012 season began when he allowed two runs and two hits while striking out seven in a career-high 8 1-3 innings of a 4-2 win over Seattle on July 4. He shut down the Mariners again Aug. 6, giving up one run in 7 1-3 innings of a 3-1 victory.
The right-hander, though, gave up two runs in three innings against the Yankees on Sept. 2 before leaving with right elbow inflammation, and he hasn't pitched since.
Manager Buck Showalter said he expects Tillman to throw another side session before facing the Mariners.
"I think he's going to have one more (side session) just to make sure it doesn't flare up," Showalter told the team's official website. "That gives us a good feeling about where he is and I think Monday will be a good spot for him."
Tillman has a 0.83 ERA while winning each of his three career starts against the Mariners, who split a six-game road trip after falling 2-1 to Texas on Sunday.
Justin Smoak homered for the second straight day and is 6 for 10 over his last three games, raising his average 10 points to .199. He had five hits over his previous 11 starts combined.
''I felt like I had good at-bats,'' said Smoak, who is hitting .194 in 11 career games versus Baltimore. ''That's all you can ask for, having good at bats, squaring balls up.''
Seattle will look to bounce back with help from Hector Noesi, who will make his first start since allowing four runs in five innings of a loss to Tillman and the Orioles in July.
Noesi (2-11, 5.54) lost five straight starts before being sent to Triple-A Tacoma following that defeat. The right-hander, though, was recalled Sept. 4 and has thrown two scoreless innings in each of two relief appearances, most recently Thursday versus Toronto.
Noesi said he has a different attitude since rejoining the Mariners.
"I think it's my position and I love to be a starter," Noesi told the team's official website. "I was being lazy before, so now I'm being more aggressive and throwing my pitches."