Marc Leishman fired a seven-under 65 to capitalise on Jon Rahm's final-round errors and win the Farmers Insurance Open by a stroke on Sunday.
Leishman produced eight birdies at Torrey Pines, including one at the last, to claim his fifth PGA Tour victory.
Overnight leader Rahm squandered his one-stroke advantage in a poor first five holes that saw him drop four shots.
The Spanish world number three charged back into contention with a strong run beginning at the 13th and threatened to force a playoff.
Leishman's birdie at the par-five 18th left 2017 champion Rahm needing an eagle to be a chance of winning the tournament for the second time.
Rahm's putt pulled up before the cup to hand his 36-year-old opponent a first Tour triumph since the CIMB Classic in 2018.
"Number five, that sounds pretty good, and on Australia Day," Leishman, who finished 15 under, told reporters.
"Playing well helps but I didn't actually hit it that well off the tee today. Putting always [matters]. You're not going to win Tour events if you're not putting well and I putted as good as I've probably ever putted today."
"This feels pretty amazing. I wasn't expecting this at the start of the day," he added.
Rory McIlroy carded a three-under 69 to finish in a tie for third with Brandt Snedeker (68) at 12 under, while Tiger Woods (70) was a further three shots back.
Woods, who learned about friend and NBA great Kobe Bryant's sudden death after completing his round, congratulated Leishman on the victory with a hug and a handshake.