The Wildcats (2-0) will take on Tulsa in Friday's winner's bracket matchup. The winner of that game will advance to Sunday night's championship game.
Shurna had 21 of his points in the second half as Northwestern charged back from 14 points down and built a 76-67 victory with 5 minutes to go.
But the Tigers (1-2) narrowed the lead to 83-82 on Ralston Turner's 3-pointer with 47 seconds left. That's when Sobelewski connected from the right corner to extend Northwestern's lead.
Johnny O'Bryant III led LSU with 21 points and eight rebounds.