Bassitt threw 25 pitches, all fastballs, to teammates Mark Canha and Adam Rosales, tossing another 25 in the bullpen. He'll do something similar in Stockton on Sunday, progressing toward a Minor League rehab stint sometime in May.
"The road to get to that for a guy rehabbing from an injury like that is difficult," A's manager Bob Melvin said. "To finally be able to get on a mound and throw to hitters and go through the incremental process to know that you're healthy enough to do it and let it go, it's a big day for him."
The A's could have the 28-year-old Bassitt back in their rotation as soon as June, just 13 months after his surgery.
"We're looking forward to getting him back," Melvin said. "He's a talented guy and was really just kind of coming into his own again when he did get hurt, and he's potentially an important guy for us in the rotation.
"There are certain guys that just look like a very uncomfortable at-bat, and he is one of those guys."
Bassitt's delivery has always been complex, and he's used the lengthy rehab time to simplify it. He's also drawn advice from pitchers who have gone through the Tommy John trial, including A's reliever Ryan Madson.
Madson, too, stands tall like Bassitt, who has enjoyed talking to a pitcher with a similar body type.
"I'm learning more and more about mechanics," he said. "I can't really get advice from smaller guys. You try to talk to Sonny [Gray] about pitching mechanics and all that. The dude can do whatever he wants to do. His feel for pitching is so good. For me to talk to him is kind of stupid, because the stuff he can do I have no chance of doing. So I've learned to take advice from people that are more like me physically, and it's translated so far."
Bassitt, obtained by the A's with catcher Josh Phegley and shortstop Marcus Semien in the December 2014 deal with the White Sox, has a 4.18 ERA in 23 games (18 starts) with Oakland.
Jane Lee is a reporter for MLB.com. Follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.