The Royals had just notched two victories at San Diego, but Seattle is where their bats have come to die. They haven't scored in their last 27 innings at Safeco Field.
Iwakuma gave up four singles and no walks in his precision performance, getting seven strikeouts.
"Obviously, he has great stuff," said the Royals' Alex Gordon, who struck out twice before getting a single. "His numbers speak for themselves. I could speak for myself, in my first two at-bats, I got behind in the count and he's got a pretty good changeup and a split-finger that's pretty good. So you can't get behind on him and expect to be successful."
The Mariners' only two hits came in the third inning against Royals left-hander Danny Duffy.
Mike Zunino led off with a double to the left-center-field wall and was bunted to third by Michael Saunders. After Stefen Romero struck out, Royals manager Ned Yost ordered Robinson Cano walked intentionally despite the left-vs.-left matchup.
"Cano's hitting .340 over his last 16 games, he's got five game-winning RBIs and I think he's one of the top hitters in the American League," Yost said.
Not only that, but previously Cano had been 3-for-6 with a home run against Duffy.
"You kind of go into it, seeing how he's swinging the bat," Yost said. "His first at-bat, he hit a pretty good curveball to the warning track in left and I just wasn't going to put him in a position where he could beat us. I figured it was going to be a low-scoring game and Iwakuma is always tough against us and he was on his game. If we were going to get beat, Robinson Cano wasn't going to do it."
Next up was Corey Hart, who played for Yost when he was managing the Milwaukee Brewers.
"You take your chances with Corey Hart, even though he's a good hitter, too," Yost said. "It didn't work."
Hart foiled the strategy by drilling a single to center field, scoring Zunino. Duffy got the third out but the Mariners had a 1-0 lead and that turned out to be enough.
Hart got a kick out of getting the game-winning RBI against his former skipper. He'd seen his share of intentional passes in front of him during those Brewers days.
"I've played with Prince [Fielder] and [Ryan] Braun and those guys and it happens," he said. "I was kind of surprised with a lefty, but I gave him a little point into the dugout after I got the hit because I played with those guys, so that was fun. I liked the RBI, but I like doing it against old managers. I had a little extra motivation."
Duffy said that Hart hit a slider.
"I hung it a little bit and, with him, he's really a good mistake hitter, especially on breaking balls," Duffy said.
Duffy pitched into the seventh but when he started the inning by hitting Kyle Seager with a pitch, he was taken out of the game.
"I figured well, we'll see if we can snuff it out there and we did a good job of it. We just couldn't make it count," Yost said.
Kelvin Herrera relieved Duffy and retired six straight hitters.
In his six-plus innings, Duffy gave up one run, two hits and three walks. He struck out four, making 88 pitches (49 strikes).
"Duff did great, Duff threw the ball well," Yost said. "Got us through six innings."
This was Duffy's second start in injured Bruce Chen's spot in the rotation and he felt his command improved over his first outing of four innings. Mariners manager Lloyd McClendon was impressed.
"I faced him in Detroit," said McClendon, who was the Tigers' hitting coach. "He's got a great arm. He's got a big arm. And it seems like he's starting to harness it a little better. It was pretty impressive outing for him."
Iwakuma, making just his second start after recovering from an offseason finger injury, was looking much like the pitcher who was third in last year's American League Cy Young Award voting.
The only other time that Iwakuma faced the Royals, last Sept. 25, he shut them out for eight innings in a 6-0 Mariners victory at Seattle. That loss mathematically eliminated the Royals from the AL Wild Card race.
On this chilly night at Safeco, Iwakama had them frustrated again. Eric Hosmer singled in the first inning and again in the fourth to no avail. Japanese countryman Nori Aoki singled in the sixth inning but was left standing. Gordon got the Royals' fourth hit with two out in the seventh but was thrown out trying to steal second.
"He was good, he was mixing all his pitches. He rarely repeated any of his pitches back to back," Hosmer said.
Mariners closer Fernando Rodney got himself in trouble in the ninth but escaped with his 10th save.
Rodney began the ninth by doing something that Iwakuma hadn't done all night. He issued a walk to leadoff batter Alcides Escobar, who was bunted to second by Aoki. Then Rodney also walked Hosmer. But he struck out Billy Butler and got Salvador Perez to hit into a game-ending forceout.
That proved a bit nerve wracking for the 12,577 chilled fans, but Iwakuma got the victory he'd earned with eight shutout innings.
"Unfortunately Duffy did an even better job and we wasted a good start," Gordon said.