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INDIANAPOLIS, Ind. – Anthony Castonzo is one step closer to being in the starting lineup Sunday for the Indianapolis Colts’ season opener, or being a spectator.

The veteran left tackle was limited in practice for a second straight day Thursday, but his workload was supposed to include work with the starting unit in team drills. Wednesday he was limited to individual work. Castonzo has been out since aggravating a hamstring injury in training camp Aug. 3.

“It felt really good (Wednesday), so that’s why we’re pushing forward today,’’ Castonzo said. “As it feels good we’ll continue to push forward.

“Getting some competitive reps today, so excited about that obviously.’’

Castonzo mentioned last week it would take a “catastrophe’’ to keep him out of the lineup for opener against the Cincinnati Bengals. That nearly occurred, and wasn’t related to the hamstring.

“It was close (to a catastrophe),’’ he said with a smile. “I had a 102 fever two days earlier this week. That was close. I’m still optimistic.’’

Castonzo’s ability to make an eighth straight open-day start hinges on how his right hamstring responds to increased and steady work.

“I just have to be confident to go on Sunday,’’ he said. “Obviously today’s practice is going to be very big. Tomorrow’s practice is going to be big, too.

“Ultimately it’s going to be ‘Am I good to go?’’’

One issue that might steer Castonzo’s decision: not rushing back. That might have been the case at the start of training camp.

“It’s always easier to be smart after the last time of reinjuring it,’’ he said. “I want to make sure that doesn’t happen (again) because that was a big-time setback.

“I would say kind of learn from my mistakes. I’m going to make sure I’m definitely good to go.’’

More medical matters

Running back Marlon Mack returned to practice for the first time since suffering a hamstring injury in the preseason opener at Seattle.

He was listed as a “limited’’ participant and it’s uncertain if he’ll be available for Sunday’s opener

Wideout and punt returner Kyle Pascal also was limited with a calf injury.

Nelson debut

Quenton Nelson has made a strong first impression with the Colts.

“He’s a monster,’’ coach Frank Reich said during the preseason.

The Colts’ appreciation for Nelson was clear in April’s draft when they made him the sixth overall pick. Despite starting 36 of 37 career games at Notre Dame, the All-American guard anticipates battling his nerves Sunday at Lucas Oil Stadium.

“Yeah. It’s just something that doesn’t really go away,’’ Nelson said of pre-game jitters. “I was nervous before every game.

“After the first play, it usually goes away.’’

What won’t go away is the challenge of tackle Geno Atkins and the rest of Cincinnati’s defensive front. Nelson understands the magnitude of the moment, but insisted he’ll simply focus on his fundamentals and technique against a player who has been selected to six Pro Bowls.

Reich was certain to point out to his rookie left guard what’s waiting for him Sunday.

“Yeah, I’m excited for Quenton,’’ he said. “We made note of that with him that, ‘Hey, you get to go (against Atkins)’ . . . probably Aaron Donald and Geno and among the two best defensive tackles in the league, three-techniques in the league.

“We we’ve talked about many times, (it’s) not a one-man battle, but there are one-on-ones and Quenton will get matched with him one-on-one a fair number of times, I’m sure. So that will be fun to watch.’’

Last month, Atkins signed a four-year extension worth $65.3 million. He has led the NFL in sacks by an interior lineman in five of the last eight seasons, including last season with 9.