He missed a 2-footer on the next hole for his first bogey in four days in the Humana Challenge, then watched a short birdie try catch the edge and stay out on the eighth.
A few holes later, the five-stroke lead he took into the final round was long gone. And a few hours later, Brian Gay was holding the big trophy after an unexpected playoff.
"You're going to have your good days and your bad days, but if you live and die with every shot out there, your career is not going to last very long," Stallings said.
The two-time PGA Tour winner saved par on the par-5 14th after driving into the All-American Canal on the right side, but dropped a stroke on the par-4 16th when his 4-iron tee shot went farther than he expected and ended up in the lip of a left fairway bunker.
"I flushed it and it went right up in the lip," he said. "I didn't really have a play."
On the par-5 18th, needing birdie to win and par to get into a playoff, Stallings hit a perfect 315-yard drive to set up a 6-iron approach from 220 yards.
He took a big divot and watched helplessly as the ball landed in the left rough, rattled around the rocks and tumbled into the water. He still had a chance to get in the playoff after a penalty drop, but left himself 10 feet after a chip and missed the par putt.
"There wasn't any nerves or anything like that going into it," Stallings said. "Just hit a bad shot. Same thing that happened on 14. .. Coming down the stretch on the 72nd hole, you can't make mistakes like that. It stinks, but it's something that I'll definitely learn from."
He finished with a 2-under 70 to miss the playoff by a stroke.
Gay took advantage of a chance he didn't expect to win his fourth PGA Tour title, finding the extra distance off the tee he has longed for without sacrificing control.
Gay cracked a 300-yard drive down the middle to set up a birdie on the first playoff hole, then split the fairway with a 297-yarder and made another birdie to hold off Charles Howell III with the sun setting behind the Santa Rosa Mountains.
"I'm still in a little bit of shock," Gay said. "It kind of happened so fast there at the end the way things went down. Last year was a struggle. It was a long year, a lot of work. I just wanted to come out this year kind of refocused, recharged, and believing in myself."
Gay won on the par-4 10th, putting his 9-iron second shot 5 1/2 feet below the hole. Howell drove into the right rough, hit into the back bunker, blasted out to 15 feet and two-putted for bogey and his 14th runner-up finish.
The 41-year-old Gay, hardly an imposing figure at 5-foot-10 and 165 pounds, worked hard last year with Grant Waite and Joe Mayo to increase his driving distance.
"My whole game's been about accuracy and short game," Gay said. "I've always been a short hitter on the tour and I felt like as I was getting older I'm only going to get shorter and shorter. ... It was tough last year trying to play making those changes."
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Gay closed with a 63 on PGA West's Arnold Palmer Private Course to match Howell and Swedish rookie David Lingmerth at 25-under 263. Howell shot a 64, and Lingmerth had a 62.
Lingmerth dropped out on the first extra hole -- the 18th -- after hitting his approach into the left-side water and making a bogey.
Howell tied for second a week after opening the season with a third-place tie in Hawaii at the Sony Open. He won the last of his two tour titles in 2007.
"Anybody that says that golf is fun or whatever, has really not done it for a living," Howell said. "I would never characterize this as fun. It's different than that. It's awfully challenging mentally."
Gay began the round six strokes behind Stallings.
"The thoughts were, 'Just be aggressive, shoot as low as you can,'" Gay said.
After birdieing nine of the first 13 holes, Gay closed with five straight pars. On 18, he hit into the right greenside rough, chipped past the hole and missed an 8-foot birdie try.
"I was happy to be in the playoff," Gay said.
Given a second chance, he outlasted Howell for his first victory since the 2009 St. Jude Classic. Gay also won the 2008 Mayakoba Golf Classic and 2009 Verizon Heritage.
Playing in the second-to-last group, Howell had a chance to pull ahead on the final hole of regulation, but left his approach about 85 feet short and three-putted for par. His 5-foot birdie attempt veered left inches from the hole.
"Going into the day, I didn't really think that anybody had a chance apart from Scott," Howell said. "He's won before, he hits it long enough to take advantage of the par 5s. At 22 under, I figured if he shoots 6, 7 under, he's really not catchable. So, then to have a chance there in regulation, that's where I really would like that one back, that three-putt."
Making his second career PGA Tour start, Lingmerth hit his 4-iron approach left into the water in the playoff. He had an awkward stance with the ball above his feet.
"I didn't feel that comfortable over it, obviously," Lingmerth said.
Phil Mickelson had a 66 to tie for 37th at 17 under in his season debut.
"I was rusty starting the year," Mickelson said. "I had a great four days here where I can work on my game with perfect weather and wonderful golf courses, where I could build some momentum. Heading into San Diego, I feel a lot more confident."
DIVOTS: James Hahn eagled the 18th for a 62 to tie for fourth with Stallings. ... Russell Henley, the Sony Open winner last week in his first start as a PGA Tour member, tied for 56th at 15 under after a 69.
Press; By JOHN NICHOLSON]
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