Prior had the session with pitching coach Larry Rothschild in Mesa, while the rest of the Cubs traveled to Tucson to play the Arizona Diamondbacks. Prior had been slotted to start an intrasquad game Thursday, but that has been pushed back after Tuesday's session.
Rothschild said they worked on a few things, including getting Prior to finish his pitches and improve his balance.
"I wasn't pushing for velocity until the end, and he let it out then," Rothschild said Tuesday. "The ball came out good."
Prior did throw a lot of breaking balls in his session. He has struggled in his two Cactus League outings with his breaking ball, and had yet to throw one for a strike. In two games so far, he's given up seven runs on eight hits and five walks over 3 1/3 innings.
Last season, Prior was limited to nine starts because of problems, mainly with his right shoulder. Rothschild has worked with Prior since 2002 when the right-hander joined the Cubs. He knows him better than anyone.
"It's tough when you go through what he's gone through in the last two years with the injuries with the collision [in 2003] and the laxity in his shoulder and the broken elbow [in 2005]," Rothschild said. "I don't think it's unusual. Pitchers will go through tough times.
"I think Mark realizes where he is," he said. "If he keeps pushing it that way, it'll all come together at some point. We'll just keep working and trying to get him over the hump."
The Cubs still have some decisions to make in the final 2 1/2 weeks of Spring Training, and the fifth starter is one of them. Who wins the job most likely will not be determined until the final week.
"I think to give Prior the opportunity, it will take that long," Cubs manager Lou Piniella said. "If he's ready, we'll see. We've got [Wade] Miller, who has been throwing the ball well. We've got [Angel] Guzman, who has been throwing the ball well."
Prior, Miller and Guzman are all ahead of left-hander Neal Cotts in the race for the job. Cotts, who hasn't been as consistent with his fastball as Piniella would like, will get a chance to go at least four innings in a game this spring.
"It's early in camp," Piniella said. "As long as I've got four of my five set, the fifth spot, there's no need to hurry."
The rotation for the Cubs' first four games of the regular season will be Carlos Zambrano, Ted Lilly, Jason Marquis and Rich Hill.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.