Thornton, who agreed to a Minor League deal with the Padres on Thursday, faced hitters for the first time in camp on Sunday. He'll do so again on Tuesday and could get in a Cactus League game as soon as Friday.
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"I just wanted to see hitters," he said. "I threw nine bullpens before I got here and then a bullpen as soon as I got here, so my arm feels good. I wanted to see hitters in the box -- one lefty, one righty.
"I was a little rusty at the start, but as it wore on, I started to make better pitches and get on top of the ball. It felt good."
Even though he's in camp on a Minor League deal that comes with no guarantees, the Padres are expecting the 39-year-old lefty to make the 25-man Opening Day roster.
San Diego manager Andy Green, who is one year younger than Thornton, was impressed with his live session on Sunday.
"He's solid," Green said. "He's a guy who has been around the block a few times."
The Padres didn't have much success against left-handed hitters, especially out of the bullpen a year ago, and could be faced with some interesting choices in terms of left-handed pitchers when it comes time to settle on their 25-man roster.
Drew Pomeranz, who threw against the Brewers on Monday, will be stretched out this spring to start, though he could always go to the bullpen if need be. Another lefty, Ryan Buchter, has impressed the staff early in camp as well.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.