Summary Casey had been eliminated on the previous hole, Riviera Country Club's testing par-four 10th.
Los Angeles (AFP) - James Hahn rolled in a 24-foot birdie putt to clinch his first US PGA Tour title in a playoff over Paul Casey and Dustin Johnson at the Northern Trust Open.
After draining the putt at the third playoff hole, the par-three 14th, South Korean-born American Hahn could barely watch as Johnson missed his effort from 12 feet on Sunday.
England s Casey had been eliminated on the previous hole, Riviera Country Club s testing par-four 10th.
In the best shape off the tee, Casey s second shot from a groomed area below the green left him 13 feet and he missed the putt.
Johnson and Hahn, both in deep greenside rough, chipped closer, Hahn draining his nine-foot birdie putt before Johnson made his own four-footer.
"Amazing. I birdied two playoff holes, against Paul Casey and Dustin Johnson two, great competitors," Hahn said. "I m lucky to be here now."
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Hahn s breakthrough win comes three weeks before another life-changing event, with his wife due to give birth to their daughter.
"My golf life changes, but more importantly, I m going to have another addition to the family, and I m just looking forward to it," said Hahn, whose previous best finish on the PGA Tour was a tie for third at Pebble Beach in 2013.
Hahn, perhaps best known previously for his Gangnam Style dance at the Phoenix Open, has now booked a Masters berth and a two-year exemption on the PGA Tour.
"Obviously winning this tournament is the best thing that s ever happened to me. But in about three weeks, I m sure there s going to be another reason why I m going to be even more happy and more grateful," he said.
Casey was the first of the leaders in the clubhouse on six-under, one shot off the lead on the course after closing bogey-birdie-birdie-bogey.
Johnson missed a nine-foot birdie putt at the final hole of regulation.
"I played great today, I really did," Casey said. "I m not disappointed in any shape or form."
He could only take his hat off to Johnson and Hahn for their shots from the rain-soaked rough at the second playoff hole.
"Those guys played phenomenal recoveries, absolutely brilliant, both of them," Casey said. "Dustin s was the toughest of the bunch, shows he doesn t have just great length off the tee. He has great hands as well."
On a day when seven players held at least a share of the lead at some point, Casey, Johnson and Hahn finished 72 holes tied on six-under 278.
Casey, a 13-time winner on the European Tour who was seeking just his second US PGA win, carded a 68 while Johnson and Hahn both signed for 69s.
Spain s Sergio Garcia was clinging to a one-stroke lead with two to play, but closed with back-to-back bogeys to miss the playoff by a stroke.
Garcia finished tied for fourth with Japan s Hideki Matsuyama, rising US star Jordan Spieth, and major winner Keegan Bradley on five-under 279.
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Overnight leader Retief Goosen, the two-time US Open champion seeking his first US tour win since 2009, led by as many as two, but found two double bogeys -- at the par-four eighth and the par-four 13th -- too much to overcome.
Even two closing birdies left him on four-over 75 and tied for eighth on 280 with South Korean Bae Sang-Moon, Canadian Graham DeLaet and American Kyle Reifers.
Fiji s Vijay Singh, a three-time major champion seeking to celebrate his 52nd birthday Sunday with his first win since 2008, saw his hopes evaporate for good with a bogey, double-bogey and bogey at 15, 16 and 17. He finished with a 72 for 281.
For Johnson, it was another strong showing in his third tournament back since a self-imposed leave of absence to deal with personal issues.
He was tied for fourth at Pebble Beach last week.
"It s good to be back and good to be in the hunt and having a chance to win," Johnson said.
