The County go into day three in trouble, struggling on 21 for two in reply to Kent’s massive 519 for nine declared.
They lost opener Ben Curran and Emilio Gay under cloud cover and the Wantage Road floodlights in what was a testing period.
The sun had been shining earlier as, after a morning delay due to rain, Kent built on their overnight score of 286 for two.
Northants made early inroads as the visitors were reduced to 389 for six, but half centuries from skipper Jack Leaning (62) and number nine Grant Stewart (61 from just 44 balls) put Kent in total control.
And all this after Northants had won the toss and bowled, straight from a two-and-half day stint in the field against Warwickshire at Edgbaston.
"I can't fault our lads they kept running in and weren't getting much assistance from the pitch,” said Sadler.
“Fair play to Kent they put themselves in a position where they could have a little slap at the end.
“You get tired bodies and tired minds in four-day cricket when you have done 140 overs fielding.
"The lights came on and there was a bit of overcast when we came out to bat which isn't what we would have wanted.
“But that is the game and Division One cricket, it is relentless.”
And the head coach now wants his batters to do what Warwickshire did to Northants last week, when they batted out for the draw after the County had rattled up 597 for six after being inserted.
“We've got to bat well on Saturday and do what was done to us last week,” admitted Sadler.
"There was no real change from Thursday in the pitch, it is easy paced.
“We need to get their bowlers coming back for their third and four spells.
“Hopefully the sun is shining in the morning and we get some good fortune. We need to bat well.”
Northants will also be hoping for good news on wicket-keeper Lewis McManus who had to leave the field after injuring a finger dropping a sharp chance late in the Kent innings
The visitors meanwhile are relishing being in charge of a game after what has been a tough start to the season.
"I'm really happy,” said Leaning.
“It is nice to be on the reverse side of it at the moment.
“Sticking 500 ourselves and having them a couple down overnight is very welcome.
"I felt scratchy to start with and got more fluent as I went through my innings. I would have liked a few more but from a team's perspective we are in a great spot and chipped in.”
On Stewart’s quick fire 50, with the Kent man hammering five sixes, Leaning added: "Grant has been working on his striking a lot with T20 coming up and he got an early work out with it!
“Hopefully we see a lot more of that in the coming weeks.
“That hitting isn't even surprising, we see it all the time in the nets so when he does it in the middle it isn't a surprise anymore.
“He's a big bloke and strong guy so he clears any boundary with ease."
Northants will resume their innings at 11am on Saturday, with skipper Ricardo Vasconcelos (10no) and night watchman Matt Kelly (0no) at the crease.