La Jolla - A dip in the water at 18 didn't dampen Jon Rahm's spirits on Thursday as he launched his Farmers Insurance Open title defence with a four-under-par 68. 

The Spanish star, who could supplant Dustin Johnson as world number one with a victory this week, was three shots off the first-round lead held by Tony Finau. 

He was delighted to be there after saving par at the par-five finishing hole on the South Course.

"I hit a really good drive and it ended up in the semi-rough a little deep and you never know how it's going to come out of there, right?" said Rahm, whose shot out ended up in the water. "Just unfortunate it happened, but I made a good putt, had a couple really good putts down the stretch and had a good finish. 

"Good momentum going to tomorrow." 

Rahm knows a thing or two about momentum. He arrived at Torrey Pines for his defence of his first US PGA Tour title have just grabbed his second at the CareerBuilders Challenge on Sunday.

He said he didn't sleep a wink on Sunday night, and his manager had to intervene to help get his focus on the task at hand at Torrey. 

"Tuesday I had this day planned and my manager realized that I was a little too active, a little too amped up," the 23-year-old said. "They told me listen, you still have the adrenaline way too high, you need to focus on this week and cut the whole practice in half. 

"We changed the press conference, played nine holes and got off the golf course as soon as possible to relax. Yesterday I just watched a movie, flipped the switch and got at it again." 

Rahm said his first three holes, with a birdie at the second and birdie chances at the first and third, were "almost a dream start". 

He made the turn at one-under and birdied the 10th before a three-putt bogey at 12. 

That was quickly followed by three birdies in his next four holes. 

"I'm really happy about the way I finished," Rahm said. "You know, the last six or seven holes I hit the ball really well. The front nine was a little shakier. I hit a couple of good shots, I made good strokes, but felt just a little bit off in general. Hit a couple bad iron shots that I could have given myself birdie opportunities, but this is golf, right? It happens. 

"(I'm) really, really pleased that it ended up the way it did."

Leading first-round scores on Thursday in the US PGA Tour Farmers Insurance Open on the North and South courses at Torrey Pines (USA unless noted; par-72)

65 - Tony Finau

66 - Ted Potter Jr., Ryan Palmer

68 - Hunter Mahan, Francesco Molinari (ITA), Julian Suri, Peter Uihlein, Tom Lovelady, Rory Sabbatini (RSA), Jon Rahm (ESP), Bae Sang-moon (KOR), Grayson Murray, Patrick Reed, Roberto Diaz (MEX)

69 - Robert Garrigus, Danny Lee (NZL), John Huh, Matt Every, Matt Jones (AUS), Nick Watney, Russell Knox (SCO), Patrick Cantlay, Charles Howell, Shane Lowry (IRL), K.J. Choi (KOR), Justin Rose (ENG), Brice Garnett, Seamus Power (IRL), Michael Kim, Luke List, Jhonattan Vegas (VEN), Bill Haas, Andrew Putnam, Ben Silverman (CAN)