Carlos Dotson had 19 points for the Catamounts (19-11). Onno Steger added 13 points.
Mercer totaled a season-low 27 points in the second half.
Kamar Robertson had 17 points for the Bears (17-15). Djordje Dimitrijevic added 11 points. Ethan Stair had nine rebounds.
Western Carolina advances to face top-seeded ETSU in the first of two semifinals Sunday.
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