The larger responsibility is quite a confidence boost, and Brock is determined to help the 49ers (10-4) keep winning as they try to make another run at the Super Bowl a year after a 34-31 loss to Baltimore in the title game.
"You still have to go out there and perform," Brock said. "Not money wise, I just want it all, I want more. I want to go out there and be one of the best. That's what I fight every day for."
The 25-year-old Brock took over as a regular starter in a Week 12 Monday night game at Washington on Nov. 25, replacing the injured Tarell Brown.
Brock had been a reliable playmaker on special teams, and has transferred that success to the defense.
"I'm happy with myself, but I'm not content with it," Brock said. "It's the hard that paid off throughout my whole life. I want to keep it going."
He had 16 tackles and a forced fumble last season and shared the team lead with 15 tackles on special teams. His primary role was a special-teams standout until he has recently received more responsibility in the secondary.
Brock made two interceptions in 11 games in 2011. He signed a one-year tender in April for this season.
"Me and Tramaine came in together, so I always knew he could play," linebacker NaVorro Bowman said Wednesday. "He waited his time. His time is now, I think he understands that. He's taking advantage of it."
And the 49ers have shown a commitment to keeping much of the core of this defense together for years to come.
Bowman, Patrick Willis, Ahmad Brooks and Aldon Smith are signed long-term. Last November, Bowman received a five-year contract extension through the 2018 season, Willis is signed through 2016, Brooks through 2017 and Smith through 2015. Cornerback Carlos Rogers received a $31.3 million, four-year contract in March 2012.
"Any time you get a chance to play with a lot of guys for a long time, you have no choice but to get better," Bowman said. "It has helped him to be in this game for a while and get comfortable, and it's showing out there."
Also Wednesday, the 49ers placed fullback Bruce Miller on season-ending injured reserve as expected with a shoulder blade injury. San Francisco signed former 49ers fullback Will Tukuafu to a two-year deal to help fill the void.
Running back Anthony Dixon also will be called upon for a bigger role blocking and taking a bulk of Miller's snaps. Losing Miller is a big blow, as he does so many things on offense and is a key blocker for running back Frank Gore.
Dixon will continue to study with Miller as he takes on an increased role.
"You get ready to show the world what you can do," Dixon said. "I'm excited to have this opportunity and I'll try to take advantage of it."
AP NFL website: www.pro32.ap.org