BBC America's Aleem Maqbool in Chicago: "During this World Cup, Chicago has already seen crowds estimated to be more than 20,000 to watch on big screens outdoors.
"For the knockout stage, a screen as been put up at Soldier Field, home of the Chicago Bears, and with it have come American Football traditions. Ahead of the match, hundreds are 'tail-gating'; parking their cars, starting up barbecues, drinking and playing football in the car park.
"Thousands have been queuing for hours to get inside the stadium, most adorned with the stars and stripes. The chants of "We believe that we will win" and "USA, USA" keep going up.
"There are huge amounts of optimism here, and it has been building throughout the tournament. Fans here are adamant their team, whose players have become huge stars, can win and have the calibre and coaching to go much further."