"I was going 100 percent, just like a game, and I had no pain," Iwakuma said through translator Antony Suzuki. "I was pain-free with my finger. Everything looks good so far."
Iwakuma, who missed all spring with a sprained tendon in his right middle finger, had previously thrown three bullpen sessions. The next step in his recovery will be another 'pen session on Sunday. If that goes well, he will likely pitch his first rehab-assignment game on Tuesday with Triple-A Tacoma.
The hope is Iwakuma will return to the big club after two or three rehab starts by early May. Mariners manager Lloyd McClendon expects Iwakuma to throw around 90 pitches upon his return because of his efficiency.
"We just want to see him continue to progress and stay healthy, and we'll figure it as we move forward," McClendon said. "The main thing is when he comes back, we have to make sure he's ready to compete at the Major League level with all his pitches. It could take 20 innings. It could take less than 20 innings. We'll see."
Pitching coach Rick Waits liked the extension he saw from Iwakuma, as well as how he was "finishing pitches. He passed all the tests for me."
"I think I'm at around 70 percent right now," Iwakuma said. "I still need to command my pitches and get that feel back. It's hard to say right now. I need to have my first rehab start and go from there. [Friday] was good. So that's another step. I have to go one step at a time. That's how I look at it."
Christina De Nicola is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.