FREMANTLE and Melbourne face the same challenge when they meet in a crucial clash at TIO Traeger Park on Sunday: breaking through a top-three backline that is led by one of the AFL's best three key defenders.

While the ladder-leading Swans are in a league of their own both offensively and defensively this season, the Dockers (69.2 points conceded a game) and Demons (70.4) rank No.2 and No.3 respectively for scores against this season.  

It sets up Sunday's crucial clash in Alice Springs as a battle of the backlines, with leading key defenders Alex Pearce and Steven May at the forefront of two groups that have also managed injury setbacks. 

AFL.com.au spoke to an opposition team analyst with experience planning for both teams this season who said there were similarities as well as key differences in how the Demons and Dockers prevented scores. 

Occupying the corridor and forcing their opponents wide, he said, was a crucial common trait and a starting point for both. 

"Straight away that helps because, as a backline, when the ball comes in from the corridor you're not sure where to go and how you can support each other," the analyst said. 

"But when they've made sure the opposition can't bring it back through the corridor or go out to the open side, it helps their higher defenders and deeper defenders in their positioning, so I think they both do that extremely well.

"Most teams have a forward and a midfielder who would have a balance of patrolling the corridor and getting into the contest to apply pressure.

"But both Fremantle and Melbourne have got a really good balance of knowing when to hold that shape and knowing when to apply aggressive pressure at the opposition."

Key players in this role can include Alex Neal-Bullen and Kade Chandler for the Demons and Jeremy Sharp and Bailey Banfield for the Dockers, with those hard-running jobs viewed as equally crucial defensively as the back six defenders. 

Both teams are also blessed, however, with elite tall defenders in Pearce and May who are playing at an All-Australian standard and are without their usual sidekicks in Brennan Cox (hamstring) and Jake Lever (knee).

Luke Ryan has stepped up superbly this season for the Dockers to patrol the defence and prevent opposition one-on-ones, while Adam Tomlinson was terrific as Lever's replacement against St Kilda in an intercepting backline role. 

The difference in each team comes in the types of scores they limit, with Fremantle ranking as the AFL's best team at limiting opposition scores from turnover (37.7 a game) and the Demons ranking No.6 in this measure (43.4). 

"The Dockers control the ball and they're very good at finding a mark in their back third. They can craft the ball up the line and be in pretty good shape behind the ball when they go forward," the opposition analyst said. 

"They've got a really good balance of going for risky kicks or taking it easy and shifting teams to get the ball at least to their front half, and your chances of being scored against decrease with each quadrant of the ground you advance to."

For the Demons, their major strength is limiting opposition scores from clearance (25.0 points, ranked No.1), with the Dockers sitting fourth on that measure (27.5). 

The key for the Demons is the ruck dominance of Max Gawn and his ability to hit forward out of stoppages, preventing opponents from busting through with a clearance out the front and allowing his defenders set up well.

"They have good heat around the ball, and obviously they're dictating a little bit with Gawn," the analyst said. "Neal-Bullen also sits in the corridor position, so teams can't come out linking up with hands through the corridor."

The Dockers' ability to also sit fourth in this measure is a credit to their clearance-winning ability, ranking a clear No.1 in the AFL for average clearance differential (+7.5 per game). 

When it comes to other key defensive strengths, the Demons can hang their hat on being the AFL's best team at winning the contested ball in their backline, while also setting the standard for limiting opposition scores per inside 50. 

They are middle of the pack, however, for conceding marks inside 50 in an indication that they are at least forcing opposition forwards to win the ball in difficult positions to score. 

The Dockers' other strengths are a high-pressure rating in their backline, led by in-form rebounder Jordan Clark, while also sitting top four for overall inside 50s conceded, meaning their own intercept mark and intercept possession numbers are low.

"Simply put, both teams get good heat on the ball and good shape, but when there are quick plays they've got the really good one-on-one defenders that can get it done when they need to," the analyst said. "At the end of the day, that's pretty important."

Melbourne and Fremantle's backline strengths