Iwakuma allowed five runs -- three earned -- on seven hits over four innings in the Oakland Athletics' 6-1 victory over Seattle split-squad team Friday night.
"For the rotation, I need to have more effort to get the result for it," Iwakuma said through an interpreter. "For the season, I also need to work and get ready. I feel I'm getting better every time."
The 30-year-old right hander was 6-7 with a 2.42 ERA for the Tohoka Rakuten Golden Eagles last season but started only 17 games because of right shoulder troubles. He signed a one-year, $1.5 million contract with the Mariners in January.
Iwakuma had a rocky start against the Athletics. Coco Crisp reached on an error, Eric Sogard had a hit-and-run single and Josh Reddick and Manny Ramirez added consecutive doubles to give Oakland a 3-0 lead.
Collin Cowgill drove in Ramirez with two outs to complete the Athletics' four-run first.
Iwakuma settled down giving up two hits but facing only seven batters in the second and third before giving up a fifth run on a walk, a single and a sacrifice fly by Adam Rosales in the fourth.
"The difference between the first and second inning is I tried to pitch a little bit higher because they were aiming for the low ball," Iwakuma said. "Then I tried to use my curveball to change their minds and their eye levels."
Oakland starter Graham Godfrey also struggled in the first, surrendering a run before he got an out as Chone Figgins walked, went to third on a single by Luis Rodriguez and came home on an Ichiro Suzuki single to right.
After Ichiro, however, Godfrey retired the final 11 batters he faced and struck out five.
"It was just a matter of getting under the lights for the first time," Godfrey said. "I was excited. I had a couple of body parts moving in different directions."
Cowgill walked in the fourth to reach base in his 11th straight plate appearance before lining out in the sixth.
"I was aware of it but I tried not to think of it," said Cowgill, who is on the bubble for a reserve spot on the roster. "I'm just trying to play as hard as I can and help this team win."
Ichiro finished 3 for 4. ... Ramirez, who hit his first spring home run on Tuesday, flew out to the warning track in the third and struck out looking in the fifth. ... Friday was the last of three spring meetings between the A's and Mariners, but they'll be seeing each other often in the coming weeks. Oakland and Seattle open the season with a two-game series beginning March 28 in Tokyo, then come home to play twice more April 6-7 in Oakland. They also meet for Seattle's home opener on April 13. ... The Mariners lost for just the second time in nine road games. ... Sogard has reached base in each of the 12 games in which he has played. ... Oakland leads the majors with 107 runs in 15 games.