Jill Ellis feels motivation will not be a problem for the United States ahead of their last-16 matchup with Spain at the Women's World Cup, but hailed her side's humility.

USA wrapped up the group stage with three wins out of three after Thursday's 2-0 defeat of Sweden in Le Havre and will now take on Spain in the next round on Monday.

Lindsey Horan scored the fastest goal of the World Cup so far before Tobin Heath's shot deflected in off Jonna Andersson for an own goal, meaning USA scored a record 18 times in the group stage.

Three-time champions USA look among the strongest sides in the competition, though they could face hosts France in the quarterfinals, but Ellis is not getting carried away.

"This team is firmly rooted on the ground. There's a humility, the players have played the game for long enough to know you have to earn every result," she told reporters. 

"When you have a group of players with self-belief...nine times out of 10 players can be very talented to make the roster but they need that quality and it is pivotal at a World Cup. 

"Sometimes it is the mentality that is really important. To have players with self-belief makes my job a lot easier as they are certainly motivated and hungry."

USA's leading goalscorer Alex Morgan came off at halftime and Julie Ertz was an unused substitute, but Ellis is not unduly worried about the fitness of either player going into the knockout rounds.

She added: "Alex took a knock in the first half and I think it was more being smart about it, and similar to Julie who took a hip contusion. It's a zero-risk game in terms of having players available for the next round." 

Peter Gerhardsson defended his eyebrow-raising team selection, making seven changes, after defeat left his Sweden side facing a last-16 meeting with Canada on Monday.

"It was the most difficult starting XI I've ever selected as there was so many parameters to consider," Gerhardsson said. 

"We know after two matches we were done and we have complete control of the players, with the medical team, we knew who was at 100% and we knew we had a knockout match on Monday that was the most important one.

"So, on the basis of that we discussed it, talked possibly more than ever about the starting XI, the coaches and players, we want to involve them in the decisions as well. When we looked at the plan for the match and how to go about it we didn't think in advance it was important to be first or second.

"We knew when we prepared this it would be Spain, Canada or Netherlands, three different teams and we know we didn't have a lot of time between the games. Playing tonight, a bus trip tomorrow then we need to recuperate for Monday, the most important match."