It's rare in the NFL that, this deep into the season, we get to see two units that are the very best at something go head-to-head. This Sunday, football fans are lucky enough to see just that: the Green Bay Packers and their league-leading run defense will try to stop Ezekiel Elliott and the No. 1-ranked rushing offense as they clash with the Cowboys at Lambeau Field. It's the most intriguing matchup in the league in Week 6.

The numbers: How they've done it

Let's start with that Green Bay run defense. Through four games, the Packers have allowed their opponents to run for only 171 yards, an average of 42.8 per game. That's the second-fewest ground yards per game any team has allowed in its first four contests since the AFL/NFL merger in 1970, per Pro-Football-Reference.

And it's not as though teams just aren't trying to run against the Packers at all. They're facing 21.5 rush attempts per game, which is below the league average, but 2.5 more runs than the Steelers and Eagles, who have faced the fewest average rush attempts per game. Those 21.5 runs just aren't actually going anywhere, as the Packers have yielded a disgustingly low 2.0 yards per carry. That's the single-best figure for a team in its first four games since the merger.

How are the Packers doing it? Well, they're mostly just straight up dominating opposing offensive lines in the trenches.

Green Bay has stopped 32 percent of its opponents' rush attempts in the backfield, per Football Outsiders, tops in the league. Only two other teams in the last 10 seasons (2006 Vikings, 2012 Buccaneers) have stopped over 30 percent of rush attempts in the backfield. Per Pro Football Focus, they have not given up more than 19 rushing yards before first contact in a single game this season. Their opponents are averaging 0.31 yards per carry before contact through four games -- the NFL average this season is 1.56 per carry.

Football Outsiders also has a statistic called Adjusted Line Yards (ALY), which essentially attempts to sort out how much credit the offensive line should get for the running game on a per-carry basis; basically, how many rushing yards per carry does the line create, before the running back takes it from there? Through Week 5, Green Bay (obviously) leads the NFL in ALY against, with an average of 2.16 yards per carry. Football Outsiders has been tracking the statistic since 1996; 2.16 ALY against would be the best figure the site has ever tracked, and it's not particularly close.

On the rare occasions where a running back has broken through the Green Bay line, the linebackers, safeties, and corners have done an excellent job of keeping them from snapping off long runs. The longest run any opponent has this season is just 14 yards. The Vikings are the only other team that has yet to allow a 20-plus yard run.

Packers defenders, including Morgan Burnett, bring down Giants running back Paul Perkins. USATSI

The Cowboys have not been setting records for their overall running game through the early part of the season like the Packers, but they still enter this contest with a unit that is the best in the NFL. Dallas leads the league with 776 rushing yards (and 155.2 yards per game) and checks in fifth with an average of 4.6 yards per carry.

What we're really talking about with the Cowboys running game, though, is Ezekiel Elliott (and the offensive line), and he has set some records over these last few weeks. After running for just 134 yards on 41 carries through his first two games combined, Elliott has run for at least that many yards in each of the Cowboys' last three games. He ripped off 140 yards on 30 carries against the Bears, 138 yards on 23 carries against the 49ers, and 134 yards on 15 carries against the Bengals. In doing so, he became the first Cowboys rookie ever to top 100 rushing yards in three straight games, and the third rookie in NFL history to top 130 rushing yards in three straight.

His overall yards per carry figure has jumped with each passing week, and his yards per carry after contact (per Pro Football Focus) nearly has as well:

1 NYG 2.6 2.1
2 WAS 4.0 2.0
3 CHI 4.7 2.6
4 SF 6.0 3.3
5 CIN 8.9 3.1

Of course, we'd be remiss if we did not credit the group of absolute maulers clearing the way for Elliott to make his jaunts down the field.

Last week against the Bengals, the Cowboys for the first time this season started the five offensive linemen that blocked for DeMarco Murray when he led the NFL in rushing yards in 2014, and they gave what was likely their best performance of the season. With Tyron Smith back at left tackle after missing two games with a back injury, Ronald Leary back at left guard (La'el Collins, who took Leary's job last season, went on Injured Reserve after Dallas' Week 3 win over the Bears), and Travis Frederick, Zack Martin, and Doug Free in the familiar spots at center, right guard, and right tackle, the Cowboys utterly dominated the Bengals up front.

The Cowboys, led by their offensive line, have rushed for 776 yards on the ground in five games. USATSI

Elliott ran for a season-high 5.8 yards per carry before contact, per PFF, a figure that has risen every single week. Let's add to the chart above:

1 NYG 2.6 2.1 0.5
2 WAS 4.0 2.0 2.0
3 CHI 4.7 2.6 2.1
4 SF 6.0 3.3 2.7
5 CIN 8.9 3.1 5.8

Turning again to Football Outsiders, the Cowboys' offensive line leads the NFL in ALY, at 4.50 per carry. By way of perspective, they were at 4.40 ALY when Murray led the league in rushing two years ago, and no team has broken the 4.50 mark in either of the last two seasons. The Cowboys have also seen their runs get stuffed in the backfield only 14 percent of the time, fifth-best in the NFL, and in the last three weeks the figure is all the way down at 8.2 percent, which would very obviously lead the league.

Matchup: What it will come down to

It's here that we should point out that the Packers, while excellent through four games, have not exactly faced a Murderer's Row of opposing rush attacks. Their Week 1 opponent, the Jaguars, was missing starting running back Chris Ivory. Their Week 2 opponent, the Vikings, saw Adrian Peterson leave the game with a possible season-ending injury. Their Week 3 opponent, the Lions, was missing starting running back Ameer Abdullah. And their Week 5 opponent, the Giants, was missing starting running back Rashad Jennings and his backup/passing down back Shane Vereen.

In other words, they're about to face an altogether different test in these Cowboys, who not only have one of the best backs in the league in Elliott, but also the aforementioned best offensive line in the NFL.

That said, the Packers have really not shown weakness in any gaps so far this season. There doesn't appear to be an obvious area the Cowboys can exploit.

Click to enlarge. Pro Football Focus

Green Bay is allowing less than 4 yards per carry in every single running lane. Dallas, meanwhile, has had smashing success running wide outside Smith (left end), and to either side of Frederick and Martin, and not as much success trying to run to either side of Free (right tackle, right end) or between Smith and Collins/Leary.

The interior matchup between Leary, Frederick, and Martin and Mike Daniels, Kenny Clark, and Letroy Guion will go a long way toward determining who wins this battle in the trenches. Daniels, in particular, has practically declared his legal place of residence as "The Opponent's Backfield," and Leary should have his hands full in that matchup.

Guion returned last week against the Giants and was again solid against the run, while Clark has seen his snap share fluctuate commensurate with his performance on a week-to-week basis. The two of them have yet to come up against a challenge like the Frederick/Martin tandem, which is operating at the peak of its powers right now. Their combo block on Elliott's first touchdown against the Bengals last week should be shown on teaching film for years to come, and Elliott's 60-yard touchdown run later in the game was cut back into the hole between them as well:

The strength of Dallas' running game is right there up the middle, but they actually might be best off in this one attacking the edges of the Green Bay defense, especially with read-option plays. Dak Prescott has shown through the first few weeks of the season that he is more than capable of making the right decision when reading the man on the edge of the formation, and anyone who has watched Dom Capers' unit the last few years knows how vulnerable they are to the read-option, especially on the perimeter. Their crew of outside linebackers are mostly pass-rush types, and they love to come screaming around the edge to make a big play.

Using that aggressiveness against them has worked well for other teams in the past (see: Kaepernick, Colin and Wilson, Russell). The Cowboys have been able to add that read element to their running game this season with Prescott at the helm (it helps that Elliott was in an offense that used it fairly often when he was at Ohio State), and it has made them even more dangerous, especially near the goal line.

If the Packers get over-aggressive trying to contain Elliott's runs up the middle, don't be surprised if Dak pulls one and takes off around the edge of the defense. At that point, it will be up to Capers and the Green Bay defense to adjust.