Horschel leads at Torrey Pines while Rahm is tied fifth

Billy Horschel opened with a sizzling 9-under 63 to take a one-shot lead at the Farmers Insurance Open at Torrey Pines in La Jolla, California.

Playing the North Course, Horschel carded a clean round with nine birdies to grab the 18-hole lead as the PGA Tour opted for a Wednesday start to give the NFL a free run at Sunday's sports coverage in the States.

Michael Thompson shot eight-under to sit alone in second place. Kevin Tway and Germany's Stephan Jaeger shot matching 65s to sit two shots back in third.

Four golfers, including world number one Jon Rahm, are tied for fifth at six-under, three shots back.

"Oh, be good." It was.@JonRahmPGA makes the only eagle of the day on 18 at the South Course. pic.twitter.com/CmCVDsCdhT

Horschel opened with birdies on the first two holes, not bad for a guy who wasn't sure if he would be able to play when he woke up on Wednesday morning; the35-year-old said he has been dealing with a stiff upper back since the Sony Open.

"Still a little worried when I woke up this morning and it felt pretty good. Thankfully all the work we've done and everything I felt, it never got any tighter," said Horschel.

"It's actually felt like it's a little bit looser right now. I didn't know if I was even going to tee it up today when I woke up. Even when I was hitting balls, I still wasn't sure whether I was going to tee it up."

Thompson also posted a clean round, finishing just one birdie shy of the leader while also playing the North Course.

"It's shorter, I think the rough is a little bit lighter on the North, and you just know you're going to have to take advantage because the South course can beat you up," said Thompson.

"I was really quite surprised with how I played today because my practice leading up into