Whitlock, a semi-finalist last week in the World Matchplay, continued his good form to see off Priestley with a near-100 average and hitting five maximums.
The pair traded 180s in sharing the first two legs, before Priestley - after hitting a second maximum of the match - missed one dart at double top to break and Whitlock landed double 12 for a 2-1 lead.
Whitlock then stepped up a gear to reel off back-to-back 13-dart finishes and then landed double 16 for a 5-1 lead.
The two-time World Champion stepped in to keep the match alive after Whitlock missed darts for the game in the next two legs, landing double eight and double 16 to maintain hope of a comeback.
But Whitlock fired in a 180 amongst three ton-plus scores to kick off the next, and sealed the match on double five.