Merrick birdied four of the final five holes in regulation to shoot 4-under 68 and join Coles at 11-under-par 277. Coles posted a bogey-free 67 to get into the playoff.
The extra session looked like it would be over quickly as Coles stuck his approach shot 3 feet from the cup at the first playoff hole, No. 18. However, Merrick calmly drained his 25-footer for birdie to put pressure on Coles.
The Australian kicked in his birdie putt and it was off to the ninth on the Upper Course at Peek'n Peak Resort. Neither player could reach the par-5 in two. Coles had mud on his ball and came up well short in the right rough.
Merrick's second missed the green short and right. Both pitched on with their third shots, Coles was 30 feet out and Merrick 6 feet away. Coles' putt came up short giving Merrick a look at birdie and the win.
The 24-year-old Merrick, who is in his rookie season on tour, missed the putt low. After both players tapped in for par, they moved to the par-3 10th, which likely would have been the final hole as light was fading rapidly.
Coles missed the green left in the thick rough, while Merrick's tee ball stopped 22 feet short of the hole. Coles' chip shot came up short on the fringe.
Merrick ran his birdie putt two feet past the cup. Coles never came close with his 18-foot par effort and he tapped in for bogey. Merrick knocked in his short par putt for his first tour win.
'Coming off No. 9 today, I had missed a bunch of putts and I was hanging my head. John, my caddie, told me to hang in there and stay positive. I made that par putt on the par-three 13th and went to the next tee and told him I was going to birdie the next five holes.
'I didn't do that, but it was close. I can't believe this. Coming into the back nine, I didn't think I had a shot and to have it work out like this is just amazing.'
The round was delayed twice, for a total of nearly three hours, by inclement weather and was finished in near darkness.
Merrick was even par for his round through 13 holes with 11 pars, a birdie and a bogey. He caught fire down the stretch with birdies on 14, 15, 17 and 18.
Coles was already in the clubhouse ahead of Merrick. Coles birdied Nos. 2 and 3, then turned at minus-9 thanks to a birdie at the ninth. He carded birdies on 15 and 17 to move ahead of Virginia Beach Open winner Andrew Buckle and get into the playoff.
Buckle fired the low round of the day with his 8-under 64. That vaulted him from a tie for 29th into third place at 10-under-par 278. Craig Bowden (70) and Charlie Wi (69) were one stroke further back at minus-9.
Chez Reavie and Kim Felton shared sixth at 8-under-par 280. Craig Lile, Michael Putnam, Jason Caron and Nick Malinowski each posted rounds of 3-under 69 Sunday. They were joined in eighth place at minus-7 by third- round leader Andrew Pratt (73), Chris Anderson (70), Kyle Thompson (72) and Jason Dufner (71).