Merrick, who finished at 11-under 273, moved to No. 4 in the FedEx Cup and will likely qualify for his first World Golf Championship next month at Doral.
Fredrik Jacobson had a 69 to tie Charl Schwartzel (70) and Bill Haas (73) for third place.
Haas, who had looked to be in good position to repeat as champion, faded from contention after he made five bogeys in a seven-hole stretch in the middle of his round.
LPGA
CANBERRA, Australia - South Korea's Jiyai Shin won the Australian Women's Golf Open by 2 shots, finishing with a 1-under 72 to beat world No. 1 Yani Tseng of Taiwan after being tied for the overnight lead with teenage star Lydia Ko.
Shin mixed three birdies with two bogeys in a final-round showdown with Ko and Tseng, who had the best round of the day, a 7-under 66.
Ko's share of the lead evaporated quickly when she began the final day with a double bogey and a bogey. The New Zealand-based 15-year-old, however, fought back to reclaim a share of the lead after 13 holes before Shin dismissed the challenge.
Shin effectively clinched her victory on the 14th hole when, tied with Ko at 16-under, she holed a pitch from the deep rough behind a billboard for a birdie.
Ko finished 3-over for the day and 14-under for the tournament, in outright third place.
