Caught in a battle with hometown favourite Max Homa at Riviera, Rahm delivered two big moments on Sunday with a 45-foot birdie putt from the fringe on the par-3 14th and then a tee shot to two feet for birdie on the par-3 16th.

He closed with a two-under 69 for a two-shot win over Homa.

"Pretty incredible," Rahm said.

"(To win) at a golf course with this legacy, this history and hosted by Tiger Woods, is such an honour."

Patrick Cantlay (67) got within one shot on the back nine until two bogeys to finish alone in third, moving him to No.4 in the world.

Jason Day (65) was the best of the Australians at nine-under in a tie for ninth, while Lucas Herbert (77) finished two-over and Adam Scott (72) five-over.

Woods had four bogeys in an eight-hole stretch and shot 73 to tie for 45th.

RIGHT: Jason Day's resurgence continued with a ninth placed finish in California. PHOTO: Cliff Hawkins/Getty Images.

It was his first time playing 72 holes since the Masters last April as he tries to cope with leg and back injuries that will make PGA Tour appearances rare. He did not know if he would play again before the Masters.

Rahm now has five wins in his last nine starts worldwide, dating to his Spanish Open win. He has not finished out of the top-10 in his last 10 tournaments.

It was his third win in five starts on the PGA Tour this year, and he already has earned more than $9 million ($A13m) the last two months.

"That was a tough week and a tough Sunday," Rahm said.

Homa, who won at Riviera two years ago, began the final round three shots back. He quickly closed to within one, only for the Spaniard to come within inches of holing out from the fairway at No.8 for a tap-in birdie, while Homa made bogey from behind the green to slip three shots behind again.

And then it changed quickly.

After Homa birdied the 9th from 15 feet, he drove to the far edge of the 10th green and got down in two for a birdie. Rahm went well left. His pitch was short and rolled down the back of the green, behind a bunker. He pitched onto – and then over – the green into another bunker, and he had to make a six-footer for bogey.

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Two holes later, Homa took the lead for the first time when Rahm three-putted for bogey, only for Homa to give it back with a bogey from a bad tee shot.

The par-3s won it for Rahm, with his putter and a full swing. He finished at 17-under 267 and earned US$3.6 million ($A5.2m).

Rahm now has won just over $9.4 million ($A13.7m) in the last two months on Tour – wins at Kapalua and the California desert and on the classic course of Riviera off fabled Sunset Boulevard. He finished third in Phoenix and tied for seventh at Torrey Pines in his other two starts this year.

This is the fifth time Rahm has been No.1 in the world.

Rory McIlroy, who began the year at No.1 and won his first event of the year in Dubai on the DP World Tour, was not a factor for the second straight week after his 73-71 weekend to tie for 29th.