SANTA CLARA, Calif. — J.T. O'Sullivan is back in the hunt at quarterback with the San Francisco 49ers.

O'Sullivan took all the repetitions with the first-team offense today in place of Shaun Hill, who rested a tired throwing shoulder by sitting out practice.

Hill and incumbent starter Alex Smith have been sharing time with the first unit since the beginning of training camp last week. It was Hill's turn to work with the first unit today, so coach Mike Nolan gave the practice time to third-stringer O'Sullivan.

O'Sullivan did not take a snap during team drills through the first five days of camp.

In the spring, Nolan said there would be a three-way competition for the starting job between Smith, Hill and O'Sullivan. But when training camp began a week ago, the team had decided on a two-man competition between Smith and Hill.

Today's developments seemed a reversal of that earlier decision, but Nolan said he never actually counted out O'Sullivan.

"As I've said all along, we've got three quarterbacks battling for the position," Nolan said Thursday. "J.T's not back in it, he's been in it. This quarterback position is open. Whoever gives us the best chance to win is the guy."

Smith and Hill so far have come along slowly in new offensive coordinator Mike Martz's offense. When asked earlier this week about the two quarterbacks' progress, Martz said: "I think right now, it's just organized chaos, and they're just trying to learn all this stuff."

O'Sullivan spent a year in that offense last season when both he and Martz were with the Detroit Lions. O'Sullivan appeared comfortable running the offense today, completing eight of his 10 passes in 11-on-11 drills.

"I had no expectations about when my reps would come," O'Sullivan said. "I'm strictly about me being ready. If they would have said the first day to jump in there, I would have been ready. If they say a month from now, I'm going to be ready. It's about me doing my job as well as I can. It doesn't change if I'm in the first group or the fifth group."

O'Sullivan, who also played in NFL Europe, is in his seventh professional season with his eighth NFL organization. He threw the first regular-season passes of his career last year with the Lions.

Hill, a seven-year veteran who played his first four seasons in Minnesota, also threw the first regular-season passes of his career last year. He was finally given an opportunity to play in December after injuries sidelined Smith and veteran backup Trent Dilfer.

Hill surprised many observers by going 2-0 as a starter, completing 68.4 of his passes and finishing with a quarterback rating of 101.3.

That strong performance earned Hill an opportunity to compete with Smith, the No. 1 overall selection of the 2005 draft, for the starting job this season. O'Sullivan was signed as a free agent during the offseason.

Hill said he is not injured and his absence the past two days has been strictly to rest his shoulder. He did not throw at all Thursday, but is scheduled to return to practice Friday.

"It doesn't hurt," Hill said. "I feel fine. It's just noticeably tired. (Coaches) felt like I needed a rest, so that's what we did. It's kind of normal for this time of year. Were ready to go now."

While Smith and Hill remain the apparent front-runners, Nolan indicated O'Sullivan might be more than just a long shot for the job.

"I see a guy that's bounced around the league a little bit, but there's no question to me that J.T. O'Sullivan is tough, he's competitive and he's resilient," Nolan said. "Now, if he can play quarterback, that would be great."