Gareon Conley, the Raiders’ first-round pick, has not seen the field at training camp because of shin splints. Another cornerback, Sean Smith, who started 15 games last season, has been taking second-team reps the past few practices.

Yes, there is opportunity on the vines in Napa, and Breon Borders is working hard in the field to earn a living.

The undrafted rookie from Duke was doing so well against the second- and third-teamers at practice last week that head coach Jack Del Rio tried him with the first team.

Borders hasn’t missed a beat. Derek Carr didn’t embarrass the rookie, who has come in on passing downs alongside corners David Amerson and TJ Carrie.

“Some guys just come in with a serious, no-nonsense approach to their game and their preparation,” Del Rio said of Borders. He’s “very conscientious, works very hard. I think Breon is having a nice camp.”

At 6-foot, 189 pounds, Borders looks the part. He is smooth in coverage, and Del Rio said Borders has “some competitive grit to him.”

His good start is particularly impressive because Borders didn’t play inside against slot receivers in college, only lining up close to the sideline against bigger receivers.

He misses the sideline like a long-lost college buddy.

“As an outside corner, I had the sideline to help me,” Borders said. “When I’m on the inside, the receiver has a two-way go, he’s a super-quick receiver, and I’ve got to account for the run, too. It’s a challenge.”

So far, so good, as Carr said Borders jumps out when he watches practice video.

“He’s really proven himself,” Carr said.

Borders had 148 tackles, seven sacks and 12 interceptions in his four years at Duke, missing the last three games last year with a hip injury. Scouts didn’t think he had the top-end speed or tackling ability to be worthy of an NFL combine invite or draft pick.

Which has led to a nice surprise for Del Rio and the Raiders. They will have a better idea of what they have after Saturday’s preseason opener at Arizona.

The older players on the team have been very helpful in Borders’ transition to an NFL training camp.

“I’ve had (Amerson) in my ear, Sean Smith in my ear, TJ in my ear,” Borders said. “It’s all friendly competition. There’s no backdooring each other.”

Borders knows he also has to contribute on special teams to make the roster, so he has worked hard to impress Brad Seely on kick coverage.

“As an undrafted free agent, you’ve got to get to know the special-teams coach,” Borders said.

Conley, meanwhile, joined safety Obi Melifonwu (undisclosed), defensive tackle Eddie Vanderdoes (knee) and right tackle David Sharpe (ankle) on the sideline Tuesday as the team’s top four draft picks didn’t practice.

“It looked like a veteran rest day, except it was mostly rookies,” Del Rio said.

As for the veterans, receiver Amari Cooper missed a fourth straight practice with an undisclosed leg injury, and defensive end Khalil Mack did get one of those coveted veteran rest days, his first of camp.

Vic Tafur is a San Francisco Chronicle staff writer. Email: vtafur@sfchronicle.com Twitter: @VicTafur