Sadler, who last won a Nationwide event in 1998 at Rockingham, N.C., edged Keselowski by 0.3 seconds to take the Bashas' Supermarkets 200.
The Richard Childress Racing team driver grabbed the lead with 26 laps remaining and hung tough to log his first victory in 91 starts on the circuit.
Defending Nationwide champ Ricky Stenhouse Jr. finished third, followed by rookie Austin Dillon.
Kevin Harvick led for 107 of 200 laps but settled for fifth place after nearly crashing on the backstretch following the final restart.
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