Before the start of the Copa America, John Brooks likely was best known for coming off the United States bench to score the winning goal against Ghana in the 2014 World Cup.

Two years later, Brooks seems set on becoming known more as a regular starting central defender for the Americans.

The U.S. will play in the Copa America quarterfinals on Thursday night against Ecuador largely because Brooks and the rest of his defense have been superb in the last two games, providing solidity to a position that's been in flux for years.

Brooks had a knee injury that caused him to miss World Cup qualifiers against Guatemala in March but he has started the past six games, the last four paired with Geoff Cameron. Yet even with that recent run of stability in the center of the American defense, the U.S. has used 15 different center back combinations in its last 30 games.

"I think John over the last two or three years had his ups and downs, little roller coasters like youngsters in general have, but he stayed very determined, very committed to what he is doing. ... I think he's just now realizing the sky is the limit for him," U.S. coach Jurgen Klinsmann said. "He's getting calmer. He's becoming more mature communicating with the whole back line. As a center back you need to keep a high level of aggressiveness because you are there to intimidate those strikers. So he does all those little things better and better. Doesn't mean he is perfect, but he is really maturing his game and it's fun for us coaches to watch."

The U.S. and Ecuador met less than a month ago in a pre-Copa America warmup match in Frisco, Texas, a match that was decided on Darlington Nagbe's late goal for a 1-0 U.S. victory, although Ecuador was without some of its top players.

Ecuador last reached the semifinals in 1993, when it was the host country. Meanwhile, The U.S. will be trying to reach the semifinals of the Copa America for the second time in its four appearances and do so in soccer-mad Seattle in front of what should be an almost entirely American crowd.

"I think tomorrow has the potential to be one of the best American atmospheres we've seen," U.S. midfielder Michael Bradley said.

Part of the reason the United States not only made it out of the difficult Group A but with the top spot was because of defense that allowed only one goal during the run of play. Both of the goals allowed by the U.S. came in the first half of the opener against Colombia, and one from the penalty spot.

The last time the U.S. posted three straight shutouts came in 2013 when the Americans beat Panama (2-0) and Honduras (1-0) in World Cup qualifying and added a win over Guatemala (6-0) in a friendly. The U.S. hasn't posted three straight clean sheets in competitive matches since the 2011 Gold Cup.

And Brooks was at the center of these recent defensive performances, especially when his sliding tackle against Paraguay broke up a 3-on-1 break early in the match. He was coming off a season in which he was a consistent starter for Hertha Berlin in the German Bundesliga.

"To me it's no surprise that he's starting to show his true qualities," U.S. goalkeeper Brad Guzan said. "Playing consistently in Germany the last 12 months it's a huge step. You build confidence with every game. You go through situation where you have a good game and you go through bad situations where you learn from that. You learn how to get better and you learn what you need to do to get better."

Because of those moments Brooks has already had in his national team career, it's sometimes easy to forget he is only 23. But his youth also brings the belief Brooks can become a fixture as the central defender for the Americans for many years to come.

"The last couple of weeks (he's) becoming more consistent in everything that he does," Klinsmann said. "What he does on thee training field reflects on what happens in the game. But also just learning at that age, 21, 22, 23, they learn how to live a professional lifestyle. It's a big learning curve."