DeJesus made a layup and a dunk amid a 16-3 run the Knights used to open the second half and pulled away after trailing 47-46 at halftime. UCF led by as many as 16 the rest of the way.
A.J. Davis grabbed 15 rebounds, Chance McSpadden scored 13 points and B.J. Taylor added 12 points and four assists for the Knights, who shot 53.6 percent from the field to 41.1 percent for the Bears. Davis and Chad Brown had three blocks apiece.
UCF has won 14 of its last 15 season openers.
Ria'n Holland scored 21 of his 31 points in the first half for Mercer and made a career-high 6 of 11 3-pointers. Jordan Strawberry and Ethan Stair scored 12 apiece.
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