First came a mini-collapse by John Rollins, a combination of bad swings and bad breaks.
Then it was a 40-foot birdie putt across the 16th green by Watney, who remembered the severity of the slope at the back of the green from when he three-putted at the U.S. Open last summer at Torrey Pines.
Watney never took the lead until the final shot of the tournament, a 2½-foot birdie putt to complete a five-shot comeback for his second career victory.
"If you're going to lead for one hole, this is the time to do it," he said.
Watney, who appeared to be along for the ride for most of the final round, birdied two of his last three holes and didn't drop a shot over the final 11 holes in closing with a 4-under 68 for a one-shot victory.
No shot was more important than his hybrid from 235 yards on the par-5 18th that easily cleared the pond and settled on the top shelf of the green. He calmly rolled his long eagle putt down the slope to just inside 3 feet, and knocked in the winning putt after Rollins failed to get up-and-down from the bunker.
Watney started to deliver a roundhouse fist pump, similar to when Tiger Woods made birdie on the 72nd hole of the U.S. Open to force a playoff in June. But that wouldn't fit his understated personality.
"I'd like to imitate pretty much everything Tiger does," Watney said. "But that was not planned at all. I was more worried about the 3-footer. I just hope the fist pump looked halfway decent."
Rollins let this one get away.
He knocked in a 20-foot eagle putt on the 13th hole to restore his three-stroke margin and appeared to have the tournament in hand. But he hit a poor chip that led to bogey on the 14th, had a plugged lie in a bunker on the 16th that was so miserable he had to scramble to escape with bogey, then pulled his second shot on the 18th into a bunker, missing a 12-foot birdie putt.
"It's unfortunate that I came in with a three-shot lead and couldn't get the victory," Rollins said. "But as I said yesterday, if somebody came out and played a great round of golf and came out on top, then my hat's off to him. And that's exactly what Nick did."
Watney finished at 11-under 277 the same total Ryuji Imada had last year in the Buick Invitational when he was runner-up by eight shots to Woods, whose knee and game was not ready to defend a title he had won the previous four years.
Lucas Glover and Camilo Villegas also had their chances.
Glover briefly was within one shot of the lead and was still in the mix until he pulled his tee shot on the 17th into rough so thick that he had to hit out short of the green and missed a 7-foot par putt. He birdied the last hole for a 68 and tied for third, two shots behind.
Villegas was within one shot until a three-putt from 10 feet on the par-5 sixth, and Rollins' collapse offered the Colombian another chance. But he hit out of the rough and over the green at the 17th to make a bogey, and his 40-foot eagle putt on the 18th hole stopped a few inches above the hole. He closed with a 72.
"A little bogey on 17 kind of made it tough on me," Villegas said. "I knew I had to make the putt on 18, and I gave it a great roll."
Watney was five shots behind going into the final round and was an afterthought most of the day, never getting closer than two shots until the tournament swung in his favor on the 16th his 40-foot birdie from the fringe, Rollins' bogey from a plugged lie in a bunker.
They traded pars on the 17th, and both were in the fairway on the 18th, in range to go for the green in two.
"In a tournament like this when it's really tight down the stretch, you've got to take it. You've got to grab it," Watney said. "John is a great player, very seasoned, and he wasn't going to give me anything."
Rollins pulled his hybrid into the left bunker, and the best he could do from a tricky lie was blast out 12 feet past the cup. His birdie attempt that would have forced a playoff didn't have a chance.
Matt Jones of Australia matched the low round of the week on the South Course with a 64 and finished alone in fifth at 7 under.
British Open and PGA champion Padraig Harrington broke 70 for the first time all week in his PGA Tour debut this season and tied for 24th at 1-under 287. Phil Mickelson went the other direction, posting progressively higher scores each day until closing with a 75 on Sunday to tie for 42nd. He finished at 290, his highest 72-hole score ever in the Buick Invitational.
Rollins began with consecutive bogeys, but his three-stroke margin was restored after the seventh hole, when Villegas and Watney made bogeys, and after he made his eagle on the 13th. That's when Watney remembered the lessons from his uncle, Mike Watney, the coach at Fresno State who first got him interested in golf at age 13.
"I knew we had some good holes to play, and I definitely didn't want to give up," Watney said. "I just tried to keep my head down and give myself chances, and I was able to make some putts."
Augusta native Charles Howell rebounded from his 77 Saturday, shooting two-under 70 to finish tied for 42nd place.