Jim Mallinder’s men square up the Aviva Premiership and Champions Cup holders at Stadium MK.
Saints are in desperate need of the points to further their chances of finishing in the top six.
And Hartley knows they must win the forward battle against Saracens if they are to win the war.
“What Sarries are is a very structured team, very smart, they will hurt you off turnover ball and they’ve got good set piece,” Hartley said.
“They don’t do anything too flash, but they exit all the way up to halfway, put in good, competitive kicks and they know what they’re doing.
“We’ve got to be physical with them.
“They try to get penalties at the scrum, they love to maul and they try to get that physical dominance in their game.
“It’s a bit like ourselves. When we get that right, results seem to go our way.
“Physicality is something we need to hang our hat on this weekend.”
Saints and Saracens have built up something of a rivalry during recent seasons.
But Hartley said: “There seems to be niggle with every team we play.
“Rugby is a game of confrontation, physicality and niggle.
“I’ve got a lot of respect for the Saracens team.
“A lot of them are international team-mates and I see how hard those guys work and how dilligent they are.
“I’m sure they’re no different for their club so we’re fully respectful of them, their team and the challenge they pose.
“If anything, for us looking up the table, we can learn from these guys.
“We’ll always get up for this game, we’ll get excited about it and we’ve always had good games against Saracens.
“It’s another big team to go up against this week.”
Saints have suffered agonising late defeats to Leicester and Wasps in recent weeks.
And Hartley knows his side must learn lessons from those matches.
“One week we focus on one thing and do that pretty well but then we clock off in another area and ship a few tries like we did last weekend,” the England captain said.
“The lesson we need to learn for this weekend is how to close out a game.
“We’ve had a few game management errors that have cost us.
“The key message about Leicester was that we had three missed opportunities to score tries and then last week we gave Wasps too many chances to get field position.
“We gave away too many penalties at the maul and they keep coming back.
“It’s always learning and that’s the story of our season. We’ve been close but just not quite getting over the line.”
If Saints can get over the line this weekend, they will take a step towards securing Champions Cup rugby for next season.
And Hartley said: “The priority for us is to be playing top-flight rugby next year.
“It’s all to play for and it’s a good place to be because if we were further down the table we could just clock off, but this keeps it competitive. It keeps us working for something.”