RIO DE JANEIRO – For much of the game Friday, the U.S. women's basketball team looked sloppy, turning the ball over because of carelessness or Canada's intense defense, producing a rare American game at the Olympics that was in doubt into the second quarter.

What saved the Americans in the first half and allowed them to pull away in the second half was their intensity on defense. They held Canada to 16 points in the first quarter and six in the second, buying them time to allow their stars to find their shots.

The result: An 81-51 victory for the U.S. in a preliminary game that kept the U.S. undefeated, and proud of that defensive effort.

"It was funny because the first quarter Canada scored 16 and we're over there saying, 'We're not getting this or that done,' " said Lynx coach Cheryl Reeve, one of U.S. coach Geno Auriemma's assistants. "Then you think, 'Four times 16 is 64. That's not bad defense.' But we actually got better."

Canada runs a patterned half-court offense employing plenty of physical screens and back cuts. The Americans ask their guards to pressure the ball and prevent three-point shots. They are so efficient defensively in part because they have centers Brittney Griner, Tina Charles and the Lynx's Sylvia Fowles in the paint if their guards get beat.

"We're hard to play against because we're big," Reeve said. "And I thought we got out to the ball pretty well."

U.S. and Lynx forward Maya Moore soared in for a layup and two of the 12 points she scored, leading the team.

Lynx guard Lindsay Whalen made a conscious push to be selected for the national team in the run-up to the 2012 Olympics. She didn't need to learn a new defense to play for Auriemma. She did need to find another gear.

"He really wants us guards to be active on the ball, he wants denials," Whalen said. "Pretty much everything that you're used to doing only do it a little better and a little harder, and do it for as long as you can, because you know there's a great player to sub in for you."

Lynx star Maya Moore and Diana Taurasi each scored 12 points to lead the U.S., which amassed 18 turnovers and 22 fouls.

The U.S. held Canada to 19-for-57 from the field, including 5-for-17 from three-point range, in extending its Olympic winning streak to 45 games.

"Geno doesn't accept them not playing defense, no matter what your name is, which is what I love about Geno," Reeve said. "It doesn't matter how many points you've scored. He set that tone early in this journey we're on. We had some people who are known to be scorers. And we were like, 'We can look around and see a bunch of scorers. So what makes you different? If you're going to be different because you don't do something then you're going to be over here on the bench.'

"I think that message was received by a couple of top-notch scorers. We know what the non-negotiables are, and playing defense is non-negotiable."

Moore played for Auriemma at Connecticut and personifies his defensive teachings. "He makes it very clear himself from the beginning," Moore said. "As starters, we go out and play defense and set a tone. Most of us have won enough that we know you have to play defense to win at the highest level."