The Indian right-hander followed up his 78 on debut against Warwickshire last week with a stunning unbeaten century against title-chasing Surrey at the Kia Oval on Wednesday.
Nair, who has six India Test caps to his name, closed a rain-affected day two on a superb 144 not out from 238 balls, hitting 22 fours and two sixes along the way.
His innings was the bedrock of Northants' 351 for nine, comfortably the team's best first innings effort of the season, with Tom Taylor also shining with 66 from 77 balls.
“Huge credit to Karun because that was a magnificent hundred," said Sadler.
“He’s a wonderful player and he batted beautifully. He’s got a great temperament and there is a real calmness about him.
“That has fed into the rest of the group and you can already feel his influence in the dressing room.
“Tom Taylor also played a fine innings and both he and Karun made the most of a bit of good fortune early on and their partnership really gathered momentum and put pressure back on Surrey.”
Nair’s century was the 16th first-class ton of his career, and is the joint highest score made by a Northants batter this summer, matching Emilio Gay's 144 against Lancashire.
The 31-year-old has now also surpassed 6,000 first-class runs, and is currently averaging a whopping 222 in a Northants shirt!
With the match at the halfway stage in terms of time, both teams will be hoping the weather is kind on Thursday as they chase the wins they need for differing reasons at both ends of the table.
Play is due to start at the Oval at 10.30am.
Quotes courtesy of Mark Baldwin