Fortinet Championship final leaderboard
-21 S Theegala (US); -19 SH Kim (Kor); -17 C Davis (Aus); -16 E Cole (US); -15 J Thomas (US)
Selected others:-13 M Homa (US), M Kuchar (US), C Tarren (Eng); -9 M Laird (Sco); -7 R Knox (Sco), B Taylor (Eng); -6 H Hall (Eng)
Sahith Theegala claimed his first PGA Tour victory at the Fortinet Championship as Justin Thomas' late challenge faded.
The 25-year-old American started the final round at Napa, California two shots clear and picked up three shots in his first five holes.
He finished with seven birdies and three bogeys in a round of 68.
Thomas, a Ryder Cup captain's pick, was joint second after the third round but had four bogeys in the front nine.
The 30-year-old former world number one, who is chasing his first win of the year, finished with a level par 72 for fifth place.
Theegala effectively sealed the win on the 14th hole with a superb 20ft birdie putt to pull five shots clear of South Korea's SH Kim who eventually finished two shots back in second with Australia's Cam Davis two shots further adrift in third.
"It doesn't feel real, it's probably not going to sink in for a while," said Theegala.
"But man, there was a lot of good golf, and that was probably the most fun I've had on a golf course in my life. Family and friends cheering me on, it's mind-blowing.
"This feeling is incredible and I couldn't have done it without my whole team and everyone out here. It's a team effort and for me to put it together like this means the world."
Thomas' Ryder Cup colleague Max Homa, the winner for the last two years, was two shots further back after a closing 69 alongside England's Callum Tarren and American Matt Kuchar.