Journeyman Merrick sealed victory with a rock-solid par at the second extra hole, the driveable par-four 10th, after laying up off the tee with an iron and hitting his approach to 18 feet.
Beljan missed the green long to the left after using a driver off the tee. He left his chip short and then, after a long-range approach putt from just off the fringe, lipped out with a par putt from five feet.
The duo had finished the 72 regulation holes on 11-under-par 273, Merrick closing with a two-under 69 and Beljan sinking an 18-foot birdie putt at the last for a joint best-of-the-day 67.
Swede Fredrik Jacobson finished one stroke shy of joining the playoff after agonisingly missing a five-foot putt to bogey the par-four 18th.
Jacobson signed off with a 69 to share third place at 10 under with 2011 Masters champion Charl Schwartzel of South Africa (70) and American Bill Haas (73), the overnight leader.