The Royals are four games behind in the American League Wild Card race with five games left to play.
The front-running Rays and Indians have each won five games in a row.
The light may be dimming on the Royals' postseason hopes, but it's not dark yet.
"Every game, from now to the end of the season, is important for us," catcher Salvador Perez said. "We don't think about what's happening with the other teams, we just need a win. We can't control the other teams, the only thing we can control is our game."
If they can get past the Mariners in Wednesday night's rubber game, it could set up for a last-second run over four games against the last-place White Sox.
Ervin Santana will go on Wednesday, and he's coming off a strong outing against the Rangers in which he allowed one run over 7 1/3 innings. It could be his last chance to impress the Royals, among other teams, who are considering making an offer for Santana this offseason.
The 30-year-old is finishing the second-best season of his career according to WAR, and has the best ERA (3.16) of his career.
Santana will have to be at his best against Mariners starter Hisashi Iwakuma, who hasn't allowed a run in 15 consecutive innings.
The first-time All-Star has the third-best ERA in the AL at 2.76. He blanked Detroit to defeat Justin Verlander in his last start, allowing just seven hits in eight shutout innings.
"I had everything working today and was able to command all my pitches and concentrate until the end," Iwakuma said through translator Antony Suzuki. "So yes, I would say this was probably my best game of the season."
Royals: Big Game James ready to go
If the Royals are still alive in the Wild Card race on Friday, they'll be able to turn to James Shields.
"Pencil me in," he said after tossing 26 pitches in a bullpen session on Tuesday. "I threw about 94 [mph] in the bullpen so I'm good to go."
Shields was drilled by a line drive on Sunday, but he continued to pitch eight shutout innings. In his first season with the Royals, Shields has the lowest ERA (3.21) of his career while logging 221 2/3 innings.
Mariners: Developing the rotation
Felix Hernandez has a 2.99 ERA over 198 1/3 innings and Iwakuma has a 2.76 ERA over 211 2/3 innings.
Everyone else who has pitched for the Mariners this season has a 4.90 ERA over 1,010 innings.
But there's a group of talented youngsters who have gotten some experience, leaving the future looking brighter than the present.
"It's been impressive, I think," manager Eric Wedge said. "You look at all of our young starting pitchers, everybody's had different paths but they've all shown signs of being able to do it. [Taijuan] Walker was very consistent I thought in his three. [James] Paxton's been very consistent. You've seen some back-and-forth from the other guys, but you look at [Brandon] Maurer's last two outings, that's been very impressive. I think [Erasmo] Ramirez has learned a great deal about himself. So it's been encouraging to see these young kids pitch the way they've pitched."
• Eric Hosmer has hit .320 with an .873 OPS and 16 homers since June 1.
• The Mariners have increased their home run total in each of the last four years (180 in 2013, 149 in '12, 109 in '11 and 101 in '10).