Iwakuma, who pitched in Japan the past 11 years, gave up six runs and 10 hits over four innings in taking the loss. Iwakuma will open the season in the bullpen for Seattle, but manager Eric Wedge gave him a start in his home country, and the results weren't very pretty.
"I tried to get a win this time, but I couldn't throw my pitch the way I expect," said Iwakuma, who has struggled much of the spring to show the type of pitcher he was in Japan the past decade. "It's too bad. I couldn't do it this time."
Iwakuma said he looked at video after the game and saw that his elbow was lower than normal, so he could be dealing with a mechanics issue. He insisted his shoulder was fine, which was something that bothered him for several months last season.
"I don't feel any problem with the shoulder, but for the [radar] gun, I expect more speed," Iwakuma said. "So I need to work on that.
"I couldn't keep the tempo. I checked the VCR after the game, and it looked like my elbow was down a little, mechanic-wise."
Iwakuma will be working on it from the bullpen initially, with Wedge not trusting him with one of his five rotation spots opening the season.
"I felt like he threw the ball OK, he just had trouble putting hitters away," Wedge said. "If he does that, it's a different ballgame. I still look at 'Kuma as somebody who is working his way back. That's one of the reasons we feel it's important for him to start in the bullpen, make his transition there and we'll see where it goes from there."
It was a quiet day for all three of the Mariners' Japanese players in front of 43,759 at Tokyo Dome. Ichiro Suzuki went 0-for-4 with an RBI, while Munenori Kawasaki got the start at shortstop and went 0-for-3 and was hit by a pitch.
The Mariners thus lost a pair of exhibition games to teams from Japan's Central League and now have a day off before Wednesday's Major League opener against the A's.
Oakland and Seattle will play two regular-season games in Tokyo before returning to the U.S. on Friday.
Yomiuri, which plays its home games in Tokyo Dome, took advantage of a Seattle team that didn't pitch or field particularly well in a game in which Wedge played it as a Spring Training situation, with liberal use of backups.
Reliever Lucas Luetge, a Rule 5 Draft pick fighting to make the club, gave up a double, single and walk to load the bases with two out in the fifth.
Shawn Kelley replaced Luetge and struck out pinch-hitter Taishi Ohta to end that threat, but the Giants pushed across two more runs in the sixth off Kelley and George Sherrill, with one run scoring after back-to-back passed balls by catcher Guillermo Quiroz and the other on a delayed double steal of home.
Ackley sailed a 2-2 pitch over the left-field fence for an opposite-field home run in the first inning to give Seattle a quick 1-0 lead. He added a triple leading off the third inning and then scored on a groundout by Ichiro.
Former Major Leaguer John Bowker took Iwakuma deep in the bottom of the first with a solo shot to right, and the right-hander from Tokyo wound up struggling through most of his four innings.
Reliever Charlie Furbush also gave up a home run to Yoshiyuke Kamei leading off the seventh.