Argentina v Switzerland, Sao Paulo, 1700
Argentina remain second favourites to lift the World Cup but they have struggled to convince so far in Brazil and look set for a tricky afternoon against a Switzerland side who have lost just one competitive match since 2011.
Having scraped past Bosnia, Iran and Nigeria - each by a single goal - to top Group F, Argentina know they will have to up their game and rely less on Lionel Messi's heroics if they are going to progress much further.
Will the Swiss roll over?
Having kept seven clean sheets in qualifying, Switzerland were surprisingly generous in their 5-2 defeat to France and that thumping means they have conceded more goals than any team that made it out of the group stage.
Despite Argentina having let in just three, they have looked far from resolute as every opponent they have faced has caused them problems.
At the other end of the pitch, both teams are among the top eight when it comes to total shots taken. Only three sides had more than Argentina's 46 efforts on goal during the group stage, while Switzerland were close behind on 42.
Don't let the quality pass you by
These two sides are capable of serving up a high quality encounter given the stylish passing that has been in evidence so far.
Argentina and Switzerland have been among the top passers during the group stage, each finding their teammates with 89% of passes - which is only beaten by Germany and Italy. Argentina are alone in boasting both quantity and quality, however, with Javier Mascherano pulling the strings behind their impressive array of attacking talent, they've kept the ball for fun - attempting over 400 passes more than their opponents.
Where they both may suffer is with individual errors. No team at the finals has made more mistakes in possession than Switzerland's 22 per match, while Argentina are close behind them with an average of 20.7.
It could get Messi late on
You will need to keep a close eye on this one right until the very end. After the last-gasp drama of Sunday's matches the scene is set for another late goal and these are two teams with a penchant for thrilling finales.
Seven of Switzerland's 10 goals since mid-May have arrived after the 70th minute, including Haris Seferovic's 90th minute winner against Ecuador and both of their consolations against France. Meanwhile, it took Lionel Messi over 90 minutes to eventually break down Iran in Argentina's second group game.
Switzerland are a side used to suffering late heartache at the hands of Barcelona's playmaker. When the teams last met in 2012, the match was heading for a 1-1 draw before Messi scored in the 88th and 90th minutes to complete a hat-trick.
With both sides' fondness for keeping the ball, this looks set to be an even encounter and it may take another moment of magic to separate them. Both sides were error-prone and inconsistent during the group stage, but possess creative players who can carve a chance out of nowhere should the other slip up first.
It could well be the more mentally alert side which prevails during another sweltering early kick-off.
Belgium v USA, Salvador, 2100
"I believe that we will win" may be the American fans' chant of choice as they flock to Brazil to cheer on their country but the odds are stacked against the USA proving their supporters right when they tackle a Belgium side unbeaten in competitive games since 2011.
That 13-match run Marc Wilmots' men have enjoyed includes their narrow one-goal victories over Algeria, Russia and South Korea, but while they have not dropped a point in Brazil they haven't set the tournament alight either.
With the humidity set to rise towards 80% in Salvador, Jurgen Klinsmann will be hoping the Belgians finally wilt in the heat.
Team USA need more firepower
If the Americans are to cause an upset and build on the growing excitement back home then they are going to have to create more in the final third.
Only two sides have taken fewer shots than the USA's measly 23 and they are coming up against a Belgium outfit who have conceded just one goal - the best record of any side.
Thibaut Courtois - who has never lost a game as Belgium's goalkeeper - has barely been tested across the 270 minutes they have played.
Only four keepers have faced fewer efforts than the 26 he has had to deal with. The USA will therefore have to up their game if they are going to create the opportunities necessary to reach the quarter finals.
Don't expect early fun in the sun
It is hard to imagine that this will be a four or five goal thriller. While many viewers may be surprised by Belgium's lack of goals, having imagined them as a free-scoring squad, the truth is that this tournament has echoed their goalscoring form in the last 12 months.
In their 14 internationals since they thumped the USA 4-2 in May 2013, Belgium have only scored more than two goals on one occasion and that was a pre-tournament thrashing of Luxembourg.
The USA have one of the tournament's most permissive defences though, with only Ecuador and Colombia having faced more shots at goal than their 47, so this could be the match where Belgium's shackles finally come off.
More Belgian waffle?
Belgium's group games followed a very familiar pattern with the Red Devils keeping possession but hardly threatening until the final 20 minutes - when the game opened up and every one of their goals at this tournament have arrived.
Three of those have been scored after the 75th minute, which should be worrying for the USA. Half of the goals that the Americans have conceded so far have arrived in the final quarter of an hour - most notably Portugal's last-gasp equaliser.
This mirrors a problem they experienced in qualifying when 36% of goals they conceded arrived in this period.
With both sides harbouring genuine ambitions of progress, expect a hot and hard-fought affair that could last the full distance if the USA can stay switched on for 90 minutes.
The numbers suggest that Belgium won't get a better opportunity to justify their pre-tournament hype as 'dark horses', even if they have to bide their time.