KANSAS CITY, Mo. -- Luis Castillo, fresh off signing a rich contract with Seattle, got chased as the Kansas City Royals erupted for 11 runs in the sixth inning for a wild 13-12 win Sunday that cost the Mariners a chance to move up in the American League wild-card standings.

The Mariners stayed in the third wild-card spot, a half-game behind Tampa Bay. Seattle remained four games ahead of Baltimore for that last AL playoff slot.

"We haven't played good baseball on this trip," Mariners manager Scott Servais said. "We need to get back to playing baseball the way we have all year that's allowed us to be in this position. We do have a good opportunity to make the playoffs, and the next 10 games will define where we're at."

Mitch Haniger singled twice during an eight-run fifth for the Mariners, and they led 11-2 going into the bottom of the sixth.

"We did a lot of good things offensively, had a commanding lead in the ballgame, and we let our guard down a little bit and didn't shut it down," Servais said. "I go back to the thing we talk about all the time, controlling the strike zone. We gave them an opportunity to come back in the ballgame because we didn't throw strikes."

Castillo struggled to begin the sixth, and Michael Massey homered and drove in four runs during the Royals' biggest inning since 2004.

"I just wanted to get something up in the zone and put a good swing on it," Massey said. "I wasn't trying to hit a homer; I was just trying to find the barrel."

Said Royals manager Mike Matheny: "We had good at-bats from everyone and they did it against a playoff-contending club.

"Whenever you have a game where you are down that many runs and you find a way to get into it, it shows the fight and determination of this team."

Castillo, an All-Star traded from Cincinnati to the Mariners in midseason, got a $108 million, five-year contract that begins next season. The deal was announced Saturday.

Castillo had been traded four previous times before landing in Cincinnati, where his explosive two-seam fastball and solid slider and changeup led him to the front of the Reds' rotation. Over six seasons with Cincinnati, Castillo went 47-54 with a 3.62 ERA and nearly a 3-to-1 strikeout-to-walk ratio.

Holding a nine-run lead, Castillo struck out the first batter in the Royals' sixth. After a walk and Massey's homer, Castillo was pulled after another walk.

"[The top of the fifth, with us batting] was just a very long inning," Castillo said. "It was a long inning and I was on the bench for most of it, and I think it affected me."

Servais saw the effect.

"He hasn't been as sharp," Servais said. "Balls missing location, been in the middle of the zone a little bit."

The Royals wound up combining seven hits, including a two-run single by Massey, and five walks off four pitchers to take the lead. Matt Brash (3-5) faced four batters and failed to record an out.

"We didn't throw strikes," Servais said. "You don't throw strikes in this league, you are gifting them opportunities. It started with Luis. Luis couldn't get the ball in the zone and we had to go to some guys in the pen who have been good at doing that and they couldn't do it."

Hunter Dozier finished with three hits and two RBIs for the Royals.

Jose Cuas (4-2) recorded four outs for the win. Scott Barlow earned his 24th save.

Royals starter Max Castillo, who was recalled from Triple-A Omaha prior to the game, left after a bases-loaded walk to Cal Raleigh put the Mariners ahead 3-2 in the fifth. He allowed six runs on six hits and four walks.

The Mariners finished their road schedule at 45-36, while the Royals completed their home schedule at 38-43.

Kansas City's announced attendance of 18,350 brought its season total to 1,277,776. That was 201,883 fewer than in 2019, the most recent season before COVID-19 restrictions.

The Mariners, who have Monday off, will return home Tuesday as Robbie Ray (12-10, 3.60 ERA) faces the Texas Rangers.

The Associated Press contributed to this report.