The Wisconsin Badgers advanced to the Sweet 16 by defeating Xavier 66-63 on the junior guard's step-back three pointer at the buzzer.
Koenig's ice-cold game winner came just seconds after he made a deep pull-up three to even the game at 63.
Zak Showalter drew a charge on Musketeers guard Edward Sumner on the ensuing possession with four seconds to play to set up Koenig's buzzer beater from the corner.
Koenig made six threes and scored a game-high 20 points and forward Ethan Happ had 18 points.
The Badgers trialed by nine points with 6:19 remaining in the game, but rallied for the win.