var level = svg.selectAll(".level")
    .data(data)
  .enter().append("g")
    .attr("transform", function(d, i) { return "translate(" + (i * 200) + "," + Math.pow(2,i)*matchY/2 + ")"; })
    .attr("class", "level");

var match = level.selectAll(".match")
    .data(function(d) { return d.matches ; })// <-- THIS
  .enter().append("g")
    .attr("transform", function(d, i, j) { return "translate(0," + (Math.pow(2,j)*i*matchY) + ")"; })
    .attr("class", "match");

var player = match.selectAll(".player")
  .data(function(d) { return d.seeds; })// <-- THIS
  .enter()
  .append("g")
    .attr("transform", function(d, i) { return "translate("+ (playerOffsetX) + "," + (i == 0 ? 0 : (rectY+rectGapY)) + ")"; })
    .attr("class", "player");

player.append("rect")
  .attr("width", rectX)
  .attr("height", rectY)
  .attr("class", "player");

player.append("text")
  .attr("x", 3)
  .attr("y", rectY / 2)
  .attr("dy", ".35em")
  .text(function(d) { return getPlayerByIdOrSomethingLikeThat(d); });// <-- THIS

[
  { source: {...}, target: {...} },
  { source: {...}, target: {...} },
  { source: {...}, target: {...} },
  ...
]