Megan Rapinoe put the Americans ahead and Abby Wambach and Carli Lloyd added second-half strikes to lift the United States past Colombia 3-0. Melissa Tancredi scored for a second straight match and Christine Sinclair added a pair of her own to lead Canada over South Africa 3-0.
The results put the United States atop Group G with six points, assured of advancing at least as one of two third-place teams.
Canada improved to three points in Group F, one behind Sweden and Japan.
Canada, which lost its opener to Japan 2-1 on Wednesday, will face Sweden on Tuesday in its group finale, while the Americans will finish against North Korea.
In Glasgow, Scotland, the United States dominated on possession and chances, but created few quality moments until late in the second half.
Rapinoe gave the Americans the lead in the 33rd minute, taking a lateral ball from Alex Morgan, using a few touches to get it on her right foot and curling a right-footed drive from 23 meters into the right-side netting.
In a physical match with the United States controlling play, Wambach doubled the lead in the 74th, exchanging passes with Morgan before hitting a strike from the top of the area past Colombian keeper Sandra Sepulveda.
After Wambach hit the crossbar with another left-footed rive in the 76th, Carli Lloyd tallied for the second straight match to complete the scoring in the 77th.
Colombia was held without a goal for the fifth game at the Olympics and Women's World Cup.
In Coventry, England, Canada dominated possession against Olympic debutante South Africa, but generated relative few chances until midway through the second half.
It capitalized early when Dina Matheson took a throw-in midway into the South African half, took a touch and served a slow roller just outside the six-yard box. Tancredi made a diagonal run on to it and volleyed a low ball inside the far left post in the seventh minute.
Sinclair doubled the lead in the 58th, out leaping Janine van Wyk just outside the six and heading Lauren Sesselmann's cross. The ball arced over the outstretched arm of goalkeeper Thokozile Mndwanie, stuck the underside of the crossbar and bounced over the goal line.
Sinclair followed up by pounding the ball into the back of the net to assure the goal.
The second goal appeared to inspire Canada, which began to generate more opportunities including Matheson's left-footed drive off the crossbar in the 82nd.
Sinclair tacked on the 139th of her international careerin the 86th, taking Sofie Schmidt's pass at the top of the South African defensive third, turning and dribbling between the central defenders before sending a low shot past Mndwanie from just inside the 18.
Canada continued to press and saw Jonelle Filigno's header bounce off the right post in the 88th.