Fatigue played a role in round of 16
If the group stage was about shocks, the round of 16 was what we expected.
For the first time since the switch to a 32-team tournament in 1998, the World Cup's round of 16 games were won by the eight group winners, with the eight runners-up eliminated.
How predictable, you might think. We might as well scrap the round of 16 and put the group winners straight through to the quarterfinals. Although, of course, the actual matches weren't remotely predictable.
Two were decided on penalties, three more in extra time and another with a dramatic late turnaround, the Netherlands trailing 1-0 to Mexico in the 87th minute yet somehow coming back to win 2-1. The round of 16 might have been won by the favourites, but they certainly weren't boring matches.
Looking for explanations is a problematic task, because on paper it's very simple: Teams that finished top of their group are generally better than those that finished as runners-up.
However, it's slightly more complex than that, and it's worth considering the nature of how, why and when teams topped their group, rather than simply stopping at the simple fact they did.
Three teams had essentially (not always mathematically, but realistically) won their group after the second group game: Colombia, France and Belgium.
Everyone else had to fight in their final group game. Either they hadn't yet secured their place in the knockout stage or they needed a positive result to confirm their position at the top of their group. Therefore, the majority of group winners played strong teams for their third match. Only Colombia, France and Belgium were able to rest players.
This allowed those three a significant advantage, and Colombia, France and Belgium were the most dominant teams in their round of 16 matches. Colombia and France recorded straightforward 2-0 victories while Belgium battered the United States for long periods even though they required extra time to complete the victory.
Compare the team sheets between each side's final group game and their round of 16 game and there's an obvious difference. For example, Colombia and Uruguay had completely different periods of rest ahead of their knockout match at the Maracana.
Colombia had won Group C after the second matchday, and for the final group game, a 4-1 thrashing of Japan, they essentially used a reserve side. Of the outfield players, only left-back Pablo Armero and left-forward Jackson Martinez played the entire final group game and started against Uruguay. Juan Cuadrado and James Rodriguez played 45 minutes each against Japan.
That's effectively three complete games of action. We can give them a tiredness rating of 3/10.
For Uruguay, it was an entirely different story. They needed a victory against Italy on a swelteringly hot 1 p.m. kickoff in Natal and secured it with a late Diego Godin header in a match overshadowed by Luis Suarez's bite.
Oscar Tabarez kept almost exactly the same team for the game against Colombia, with Maxi Pereira coming into the side permanently, having replaced Nicolas Lodeiro at halftime against Italy. Meanwhile, Diego Forlan replaced the suspended Suarez. Therefore, eight and a half players had played the match against Italy. Tiredness rating: 8.5/10.
The contrast was obvious. Colombia are a better side, certainly, but they were also sharper and quicker and their full-backs were capable of going on long-distance runs to push back the Uruguayan wing-backs. Tabarez's side recovered once he turned to his bench in the second half.
Similar effects can be observed when assessing the other two dominant round of 16 performances. France kept just four outfielders -- Laurent Koscielny, Paul Pogba, Blaise Matuidi and Karim Benzema -- from the side that had drawn 0-0 against Ecuador with Group E already sewn up. Tiredness rating: 4/10.
Their round of 16 opponent Nigeria were in the opposite situation. They had made only one change, and that was because of Michel Babatunde's injury. Victor Moses came into the side, but everyone else remained the same. Tiredness rating: 9/10.
Belgium were the third side to win their group comfortably and retained just Daniel van Buyten, Jan Vertonghen, Marouane Fellaini and Dries Mertens between their last group game and their round of 16 match with the U.S.
Jurgen Klinsmann selected eight of the same players for the U.S., with Kyle Beckerman and Brad Davis making way for Geoff Cameron and Alejandro Bedoya.
Tiredness rating matchup? 4/10 vs. 8/10.
This is all anecdotal evidence, of course, and the fact remains that group winners are simply more likely to be better teams than runners-up. When you throw in the fact that two matches were decided in favour of the group winners on penalty kicks, there was an element of unpredictability too.
However, in the only three dominant round of 16 matches, fatigue was a huge factor: Uruguay played Colombia on the back of just three days' rest, whereas the majority of the Colombians had enjoyed eight. That pattern was repeated for the other two similar games.
In summary, there is certainly an advantage in winning your group and (generally) avoiding a strong opponent in the round of 16, but there's an even bigger advantage in winning your group before the third matchday. It allows a crucial extra period of rest, which often proves decisive.