Dexter, ranked 8th in Class 4, held off a late Kelly rally to pull out the victory.
Kelly, ranked No. 7 in Class 3, falls to 14-2 on the season.
Essner battled foul problems all night but she still scored a career-high 36 points and had seven steals.
"What a ball player," said Dexter coach Gavin Miller. "I don't know if I've seen a guard do what she does. She can take it to the bucket or she can back up and hit the 3. From my understanding she does a little bit of everything over at Kelly. She's a great softball player, she could probably hit a golf ball if you put it in front of her or play volleyball if they had it. Those are the types of kids you want on your ballclub."
"It's nothing major, not too serious," said Kelly coach Rod McQuerter.
Dexter took advantage of Robert's absence by pounding it inside to Taylor Nelson, who scored 19 points for Dexter.
Rachel Blunt led the Lady Bearcats with 20 points.
"Taylor got us going early," said Miller. "She needs more touches if you want my honest opinion. She could probably average 15 to 17 points even with Rachel getting her 20."
Dexter (16-3) led by as much as 15 through the third quarter as they held a 52-37 lead heading into the fourth.
But Miller knew the lead wasn't safe against Kelly.
"We were up by 15 in the third and I told my girls that this Kelly team wasn't going to give up and they would fight," said Miller. "We need games like that. This is a very talented Kelly team. They've been ranked all year. We knew that we were in for a fight tonight. Good teams make you make mistakes and we did."
Kelly was able to dwindle the Dexter lead down to 58-52 on an Essner 3-pointer with 1:33 left in the game.
But Dexter was able to break the Kelly full-court press for some easy baskets to pull out the win.
"We were fortunate that a couple of their key guards fouled out and we were able to put our press on and get some steals out of it and got some momentum," said McQuerter. "But we had a couple missed assignments after that and Dexter capitalized on it. But I love my girls to death. They played their butts off."
Kelly led only in the early stages of the game. Essner had back-to-back conventional 3-point plays to give the Lady Hawks a 10-8 lead in the first quarter but it didn't hold up.
Dexter led 20-14 after one quarter and extended the lead to 33-23 by halftime.
"When you get two state-ranked teams together this is the ballgame that you want to see," said McQuerter. Dexter is a well-coached and well-disciplined basketball team. When you go against a quality team, you have to do everything right. The ball rolled their way tonight. They hit some big shots and they deserved to win."
Kelly was 16-of-20 from the free throw line, including 11-of-12 from Essner.
"It was a very good effort by Kelly," said McQuerter. "She's the perfect team player. The way she played tonight is the way she practiced every single day."
Jessica Burnett added nine points for Kelly. Haylee Finch had nine points for the Lady Bearcats.
Kelly will play Twin Rivers on Saturday at 1 p.m. at the Black River Coliseum in Poplar Bluff.
Dexter will host Charleston tonight at 6 p.m.