"I was supposed to pitch three innings, but unfortunately I threw too much," Iwakuma said through translator Daisuke Sekiba. "So [pitching coach Carl Willis] said, 'Why don't you finish up?'
"I wanted to pitch more, because I'd only pitched one inning. I wanted to pitch two or three, but they said they were thinking about me and worrying about too much pitches. But I felt pretty good, even at 35 pitches. Next time I'll try to pitch more innings to get experience with what it looks like."
Scott Patterson, George Sherrill and Tom Wihelmsen each tossed a scoreless inning for the Mariners. Adam Moore gave Seattle a 7-5 lead with a home run to start the eighth inning.
The Padres, who saw each of their first three pitchers allow runs, fell to 0-2.
San Diego starting pitcher Dustin Moseley allowed two runs, both in the first inning, in two innings of work. He looks to be the favorite to win the No. 5 spot in the starting rotation.
"I was trying to throw everything, mix everything in," Moseley said. "I thought the second inning was a little better. I wanted to go out and throw a lot of strikes, and I did."
Newcomer Micah Owings, who is in competition with Anthony Bass for the long relief job, allowed two runs on three hits with two walks in two innings.
Jason Hagerty had a two-run triple as part of a four-run sixth inning for the Padres.
Carlos Quentin had two hits, including a double, and reached base three times. Jeremy Hermida knocked in a run with a pinch-hit single.
Up Next for Mariners: Felix Hernandez makes his Cactus League debut as Seattle hosts the Reds at 12:05 p.m. PT at Peoria Stadium. Hernandez, who is being lined up to pitch the Mariners' season opener in Tokyo on March 28, has thrown one intrasquad game and one simulated game and likely will throw three innings. Shawn Camp, Hong-Chih Kuo, Chance Ruffin and Steve Delabar are all scheduled to throw in relief. The game will be on radio on a delayed broadcast at 7 p.m. PT on 710 ESPN Seattle.