FOURTH ROUND LEADERBOARD: -12 H Mahan (US) -10 R Palmer (US) -9 R Goosen (SA), B V Pelt (US) Selected others:+3 P Mickelson (US) +18 T Woods (US)
American Mahan hit a perfect round of no bogeys
Hunter Mahan hit a final-round 64 to earn a two-shot victory at the WGC-Bridgestone Invitational in Ohio.
Mahan ended on 12 under par finishing ahead of Ryan Palmer with Retief Goosen and Bo Van Pelt a shot further back.
Northern Ireland's Rory McIlroy and Irishman Padraig Harrington were seven shots off the lead on five under par.
World number one Tiger Woods had a miserable final round of 77, ending 18 over par, but rival Phil Mickelson could only hit 78 to end on three over.
Mickelson's score meant he missed on the chance of surpassing American compatriot Woods as the world number one.
But despite Woods and Mickelson's failings, the day belonged to American Mahan who held his nerve to hit six birdies and no bogeys in a faultless round that saw him earn victory.
He started the day five shots off the lead but five birdies on the front nine saw him take the lead as players initially ahead of him faltered.
And despite only picking up one further shot on the back nine, he did not give any of his rivals a look in, holding on for victory with some fine par saving putts.
"I felt great, I knew I had a chance and to not make any bogeys on a Sunday gives you a great chance to win," said Mahan.
"On the final few holes I just told myself to make pars, I didn't want to open the door to anyone.
"The best players in the world are out here, it's a beautiful golf course and it means a lot to me. It feels great, I knew I had it in me but to do it it feels unbelievable."
Victory for Mahan also saw him earn Ryder Cup selection, something he called a "huge honour."
Mahan has played on the last three US teams, always as a captain's pick. He earned $1.4m (£876,000) as a result of his victory and moved up to number two in the Ryder Cup standings with one more qualifying event.
It was his second victory of the year, having also won the Phoenix Open.
Bill Lunde rallied with a six-under 66 to finish at 17-under, holding off JJ Henry by one stroke, to win the Turning Stone Resort Championship in Verona, New York State.
The win is the first for Lunde in his second full season on the PGA Tour. He vaulted into the lead with six birdies on the front nine and played even par for the last nine holes.
It earns the American, 34, a spot in next week's PGA Championship at Whistling Straits, Wisconsin.