Belgium came into the European championships as a favorite, but its path to a quarterfinal matchup with Wales has been rocky. In the very first match of the groups, Belgium was beaten 2-0 by Italy in a match that threw into relief all of the Red Devils’ weaknesses. Manager Marc Wilmots, not considered a great tactician, had played his two best attacking midfielders in wing positions that did not suit them. Italy’s defense keyed on Belgium’s weakness in established possession and allowed Belgium to pass around aimlessly but kept it out of the open field. Since that loss, however, Belgium has dominating wins over Ireland, Sweden and Hungary by a combined eight goals to none.

The secret to Belgium’s turnaround is simple. Wilmots found a way to open up the games. He moved Kevin De Bruyne from the wing to his favored No. 10 position as a playmaker behind the striker and dropped the miscast Marouane Fellaini from that role. This move also allowed Eden Hazard more freedom to cut in from the wing and combine with De Bruyne, while adding another pacey wing to the attack in Yannick Ferreira-Carrasco.

The outcome of this change in personnel was drastic not only in real goal difference but also by the underlying numbers.

This lineup allowed Belgium to play in the open field where Belgium’s front four, especially striker Romelu Lukaku, is at its most effective. These attackers have mostly stayed high up the pitch or, when they have dropped back in defense, looked for opportunities to leak out and counterattack. This tactic has left massive amounts of space between Belgium’s lines and drawn opponents into the sort of wild, end-to-end matches where Belgium’s great attacking talent is likely to win out. The outcomes have been fantastic for Belgium fans and neutral spectators alike.

To measure how open matches are, I count the number of direct attacks a team makes into the opposition defensive zone. These are attacking moves by either team where ball movement is highly progressive (at least 60 percent directly forward to goal) which cover at least 25 yards forward. Belgium’s last three matches rate as three of the seven most open matches at the Euros this year.

In Belgium’s three matches in the new system, there have been 64 such attacking moves, 40 by the Red Devils and 24 by their opponents. The only quarterfinal teams that have allowed more than 22 deep, direct attacks over all four matches are Iceland and Wales. Belgium has opened up the game not only to the benefit of its own attackers but also to dangerous attacks from its opposition. So far Ireland, Sweden and Hungary have been unable to capitalize.

But note the fourth match in the above chart, Wales’s impressive 3-0 defeat of Russia. It is unsurprising that Wales’s best match of the tournament should be its most open. The one rule of containing Wales, which its non-Russia opponents have mostly achieved, is to prevent Gareth Bale from having space to play. The Real Madrid star is the undisputed key to Wales’s attack, and he has near-complete freedom in Wales’s shape to find space and carry the ball forward. The same open 4-2-3-1 shape with acres of space between the lines that enabled Belgium to romp through its last three matches may also provide the perfect opportunity for Wales to spring the upset.

If the game plays out this way, the two players to watch are Bale and Hazard. While Bale was putting up massive numbers for Champions League winners Real Madrid, Hazard was struggling through a lost season for Chelsea. But at the Euros, Hazard appears back on track. He was a wrecking ball against Hungary, carrying the ball through traffic and disrupting any defensive strategy the Hungarians might have played. He took an opponent one-on-one and drove by him 12 times, far more than any player had done at the tournament so far.

Belgium’s strategy in recent matches has key weaknesses that Wales should be able to exploit. But if Hazard has another spectacular match, tactical issues may be moot. Of course, the same is true of Bale. This means that if Wilmots and Belgium keep the tactics that brought the Red Devils to the quarterfinals, Friday’s quarterfinal may be settled by which of Bale or Hazard takes better advantage of the opportunity to show their stuff in the open field.