When Quinn Cook was 13 years old, he joined D.C. Assault AAU team with another point guard. That’s when he met 14-year-old Tyler Thornton.

The two played on the same team for only a year and a half, but competing in the same program made them fast friends. Not even rivalry games between Cook’s DeMatha High and Thornton’s Gonzaga High shook their friendship.

“A lot of people wouldn’t want to see us hanging out together, but we did anyway, because our friendship went before that,” Cook said. “We had a strong relationship before I got to Duke, and it’s been a blessing to have him here with me.”

Their relationship has paid dividends for No. 1 Duke (14-0, 1-0) this season, as the two form an effective ball-handling tandem. The Blue Devils’ assists-to-turnover ratio (1.48) ranks 10th in the country and first in the ACC. Cook, Thornton and the Blue Devils will host Clemson (8-5, 0-1) at 7 p.m. Tuesday at Cameron Indoor Stadium.

Thornton, along with Josh Hairston, played a role in convincing Cook to come to Duke, attempting to influence him the way Nolan Smith had impacted them. And ever since Cook stepped foot on campus, he has turned to Thornton, whom he describes as his big brother and best friend.

“It’s the constant big brother role he plays. If it’s off-the-court issues, if it’s dorm life, he’s been through everything I’m going through right now,” Cook said. “On the court, if he’s not in the game, he’ll pull me to the side in the first media timeout and tell me what he sees. I’ll ask him in walk-throughs how to defend certain things. It’s just constant help.”

By committing himself to an offseason fitness program at Thornton’s recommendation, Cook put himself in position to be Duke’s primary on-ball defender. That frees Thornton to guard off the ball and allows him to see the floor better, passing along advice to Cook and Rasheed Sulaimon throughout the game.

By helping Cook, Thornton actually hindered his own chances of becoming Duke’s starting point guard. Cook won an open competition between the two, and he has improved significantly as the season has progressed.

Cook is averaging 6.2 assists per game (with a 2.7 assist-to-turnover ratio) and 10.3 points per game. The only Blue Devils player to average at least six assists and 10 points per game are Bobby Hurley (1990, 1991, 1992, 1993), Dick Groat (1952), Jason Williams (2000, 2001) and Chris Duhon (2004). The first three have their numbers hanging from the rafters at Cameron.

Last time out, Cook had a career-high 14 assists against Wake Forest (keeping his focus despite shooting zero of 11 from the field) and just one turnover. That ratio tied for second-best in school history, behind Hurley.

Meanwhile, Thornton has become Duke’s sixth man and is the only reserve to average at least 11 minutes. He ranks second on the team in assists (38) and steals (24), but, as associate head coach Chris Collins said, Thornton’s value isn’t measured by stats alone.

“He does so many good things with the younger guys in terms of bringing them along,” Collins said. “You don’t see that as much especially with an older guy, you might not want the younger guys and the guy at your position to play as well as you, that’s just human nature.

“He’s just the epitome of a winning player. He does a lot of dirty work for our team.”

And much of that has allowed Cook to shine.

Keeley 919-829-4556; Twitter @laurakeeley