A gathering at Grant Park in Chicago was moved to Soldier Field to accommodate a crowd reported to have exceeded 25,000.
World Cups have been growing in popularity among Americans for some time, but this tournament has felt different. Explanations for the surge vary, with some pointing to Brazil's time zone being favorable for American viewers, especially compared to South Africa four years ago. Others say soccer's spike is simply the result of a growing Hispanic population in the United States as well as the inevitable aging of Millenials. A great number of soccer-loving children have now become consumer adults.
"These are all young people who grew up with the game, whether it be the English Premier League or Major League Soccer, and they don't need to be convinced that soccer is a sport that is worthy of their attention," said Don Garber, the commissioner of M.L.S. "The country has changed. This is a new America."
Statistics seem to support that claim. Fourteen percent of people between the ages of 12 and 24 said professional soccer was their favorite sport, second only to the N.F.L., according to Rich Luker, who runs a sports research firm. That means a greater number of fans are more likely to continue following the sport even when the pageantry of the World Cup is over.
"Fans are connecting the dots," said Jeff L'Hote, who runs a soccer-focused management consultancy. "One of the great things about the continued maturation of the sport is people know that Messi plays for Barcelona, not just Argentina."
Read More World Cup a tipping point for soccer in US?
Whatever the baseline theory, the sheer entertainment value of this tournament has piqued interest even more. Including Tuesday's games, 154 goals have been scored, which is more than the total for the entire 2010 World Cup. Also, for casual American fans who find watching games end in ties about as appealing as doing their taxes, the past few weeks have been a revelation: There were only nine draws in 48 group stage games, or four fewer than the average over the past four World Cups.
The grittiness of this United States team was an attraction, too. Drama, in one form or another, has followed the United States team ever since its pretournament training camp in May, starting with the uproar over Coach Jurgen Klinsmann's decision to leave Landon Donovan at home.
Then came concerns about whether the Americans could win a must-have game against Ghana (yes, barely); whether they could get a decent result against Portugal (yes, agonizingly); and whether they could avoid a blowout against Germany (yes, mercifully).
"I think every player went to his limit," Klinsmann said.
Tuesday followed a similarly tense script. Belgium, which won all three of its group games with late goals, was the aggressor from the start, neutralizing Klinsmann's attempts to open up the American attack.
Divock Origi and Dries Mertens had early chances for Belgium, and things got worse for the United States when Fabian Johnson, one of the most reliable players on the outside, pulled up with a hamstring injury after half an hour.