No one saw it coming. The way things were unfolding in late 2010 and 2011, the United States’ rival of the future, the up-and-coming team gunning to unseat the powerhouse program, was going to be Mexico. First, El Tri upset the Americans 2-1 in World Cup qualifying. Then, the teams had a testy meeting in 2011 right before the World Cup. The set up was there, but a grueling match in the Olympic Games altered that course. Now, Canada, that’s right, Canada, has emerged as the U.S. women's main rival, not just in North America, but the world.

The battle continues on Sunday, and there is still bad blood in the air.

After the Olympic semifinal, FIFA investigated Canada coach John Herdman and star forward Christine Sinclair, suspending the latter four games and fining her $3,500 for unsporting behavior towards the referee. American supporters called for a FIFA investigation into Mellisa Tancredi’s head stomp on Carli Lloyd. The teams have not played since, but the rivalry has only grown.

The relationship took another turn in late 2012 when U.S. Soccer announced the launch of the National Women's Soccer League. The American, Canadian, and Mexican federations would subsidize the salaries of dozens of national team players. That meant that the U.S. and the Canada would be playing with and against each other on a consistent basis for two years leading up to 2015 Women’s World Cup, allowing the players to learn each other’s tendencies, observe weaknesses and strengths, and develop alliances and rivalries.

It’s no secret that Canada’s goal is to win the 2015 Women’s World Cup on home soil, and everything from the NWSL to Sunday's friendly are stepping stones towards that end. But Herdman, who usually emphasizes long-term focus rather than short-term results, seems to have take exception with the coming game.

“In the past I have talked about having an absolute 2015, 2016 focus and the development steps this year,” Herdman said. “But I think for this game you’ve got to consider the significance for Canadians. We can’t hide from this. I think it’s an important game for Canada and a game we’re really interested in. When you bring in the world No. 1, a team that hasn’t been beaten in 35 games and that we haven’t beaten in 12 years, you want to make sure that you have a really strong team out there that can cope with that experience.”

Herdman has called in all of his big guns, including Sinclair, Diana Matheson, and Sophie Schmidt. That latter two women sit atop the NWSL scoring table with five and four goals, respectively. To add a little extra bite to the squad, Herdman has also summoned Tancredi, who took the year off to focus on chiropractic school.

U.S. coach Tom Sermanni is also aware of the stakes.

“It’s always an important match when you play a neighbor, and with the strong contingent of Canadian players in the new league and how close the competition has become between the two countries over the past few years, there’s an intensity about this game far greater than a typical international friendly,” he said. “I know our players are really looking forward to the match and to the crowd, but also to getting back together as a national team and continuing to evolve in a positive direction.”

The U.S. will come with a strong squad featuring all of its familiar faces except Hope Solo and Megan Rapinoe. Solo is training with the team but will not be active on Sunday, while Rapinoe is still in France with her club, Lyon.

Chances are we won’t see a repeat of last summer’s epic battle on Sunday. However, with the stakes high for both teams, at the very least it will be an engaging chapter in a rivalry that will define the future of the women’s game.

Maura Gladys, a featured ASN columnist, works in production for KICKTV. She also runs the goalkeeping blog All You Need Is Glove.