And it's a game with no shortage of contrasts.
The U.S. has no iconic figure in its team, but is coming off a morale-boosting win over a familiar World Cup foe, Ghana. Portugal has Cristiano Ronaldo and the hangover of an embarrassing 4-0 loss to Germany.
Those previous results mean there is plenty at stake in Group G for both sides.
Sunday is also Day 2 for Group H, where Belgium takes on Russia at Rio's Maracana stadium and South Korea meets Algeria.
Things to watch for Sunday:
EYES ON RONALDO
The United States will advance to the knockout stages of the World Cup if it can beat Cristiano Ronaldo's Portugal in the heart of the Amazonian rain forest.
Which Ronaldo will show up? The one who looked lost and timid against Germany, or the one who ripped defenses to shreds for Real Madrid last season, scoring 31 league goals, winning the Champions League and being voted world player of the year.
The U.S. will be hoping it's the former.
Ronaldo has been troubled by knee and thigh injuries since before the tournament, but no player in the U.S. team can afford to take their eyes off Portugal's poster boy.
The good news for the Americans is that Germany's 2-2 draw with Ghana on Saturday means a victory in Manaus would guarantee them a spot in the second round.
Venue: Manaus. Kickoff 1 p.m. local time (1 p.m. in New York, 6 p.m. in London, 2 a.m. in Tokyo).
Eden Hazard, a creative attacking midfielder who is coming off a superb season in the English Premier League with Chelsea, will be Belgium's main threat against Russia.
There weren't many of his trademark teasing runs though the defense in his team's opening 2-1 win over Algeria. But he has a chance to be a standout player at the World Cup, and a victory on Sunday will put his side into the second round.
Team captain Vincent Kompany of Manchester City has recovered from a groin strain in the first game and will return to the center of Belgium's defense.
Russia goalkeeper Igor Akinfeev was at the center of his team's first game after he let a soft shot through his hands in a 1-1 draw against South Korea. "It was a kid's mistake," he said. Fortunately for him, coach Fabio Capello says it won't cost him his place in the starting lineup.
Russia captain Vasily Berezutsky attributed his side's labored performance to first-night nerves for a national team that hadn't been at the World Cup since 2002.
"There was an adrenaline factor because no one has played at this level before," Berezutsky said. "Now everybody is calm and ready."
Venue: Rio de Janeiro. Kickoff 1 p.m. local time (noon in New York, 5 p.m. London, 1 a.m. Tokyo)
FINDING THE TARGET
South Korea probably won't get a better chance to take three points from a game at this World Cup, so expect a more attacking performance when it faces Algeria.
The 2002 co-hosts were fortunate to get a draw with Russia, but a victory would put them in a good position to qualify from Group H.
Coach Hong Myung-bo is under no illusions. "We are going to have to play a different game compared to the Russian match," he said. "There will be opportunities to score."
Algeria's players will not need reminding about the stakes, having already lost to Belgium.
The one good omen for the Algerians at the Estadio Beira-Rio is that they've already scored a goal in Brazil, albeit a penalty, having failed to find the net in all three games in South Africa four years ago.