(CNN) -- Phil Mickelson's hopes of a first victory since January were derailed by his struggles at "the Green Mile" on Sunday, as rookie Derek Ernst earned his maiden PGA Tour title in a playoff at the Wells Fargo Championship.
While four-time major winner Mickelson dropped shots at two of his final three holes, Ernst birdied his last to force a playoff with Englishman David Lynn -- which he won at the first hole.
It earned him the $1.17 million first prize and an invite to next week's $9.5 million Players Championship -- the golf season's unofficial "fifth major."
Ernst, who turns 23 on May 16, had made the cut in just two of his previous seven tournaments of his first season on the circuit, with a best finish of 47th in New Orleans last weekend.
The California native graduated from University of Nevada-Las Vegas last year with a degree in hotel management before turning pro.
"This feeling is unbelievable right now," he told reporters. "Just at the beginning of the week not even knowing I was going to be in the field. I was fourth alternate last time I heard, and then a couple people dropped out so I got in."
Mickelson has won 41 times on the tour, and seemed set for his second victory this season despite dropping three shots at 15 and 16 in Saturday's third round.
The 42-year-old birdied 14 to be in contention but then again lost a shot at 16 to be in a three-way tie for the lead as the crowd sheltered under umbrellas in the wet conditions in South Carolina.
He then bogeyed 17 -- where he is now 14 over par in the 40 times he has played that hole -- and missed his birdie putt at 18 to card a one-over-par 73.
The last three holes at the Quail Hollow club are known as "the Green Mile" -- a stretch which is rated as one of the top-five hardest finishing runs on the PGA Tour in the past decade.
Mickelson racked up his seventh top-10 finish in 10 appearances at Wells Fargo, ending up one shot ahead of England's former world No. 1 Lee Westwood and Sweden's Robert Karlsson, who tied for fourth.
Lynn, 39, had also been seeking his first PGA Tour win, having finished in a tie for fourth at the Honda Classic in March -- and runner-up at last season's closing major, the PGA Championship.
He and Ernst had been three shots behind Mickelson going into the fourth round.
World No. 2 Rory McIlroy, who won his first PGA Tour title at Quail Hollow in 2010, finished tied for 10th after carding a second successive 73.
The Northern Irishman bogeyed his first and last holes, and also picked up a double at the par-four 12th.
Meanwhile, Brett Rumford completed his second successive victory on the European Tour's Asian swing with a four-shot victory at the China Open on Sunday.
The 35-year-old Australian followed up his playoff win from last weekend's Ballantine's Championship in South Korea as he carded five birdies in the first 10 holes of his final round.
It put him top of the Race to Dubai standings, having been 138th two weeks ago.
"It's quite surreal -- it's the first time I've actually played the week after a win so I'm more than pleased. It's hard to get my head around it at the moment," Rumford said.
Finland's Mikko Ilonen was second, having started the fourth round one shot behind Rumford.
Defending champion Branden Grace of South Africa tied for 33rd in a group including 16-year-old Dou Zecheng -- who was the best finisher of the group of young Chinese talent in the field.