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.
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; the 35-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 today wasn't that great. Something clicked and I made a bunch of putts and I was hitting my lines, so nothing to complain about."
Defending champion Patrick Reed opened on the South Course and shot even-par 72. He's tied in 96th after two birdies and two bogeys. Hideki Matsuyama is right there with him after his first 18 on the South Course.
Other notables include Dustin Johnson, Justin Thomas and Xander Schaufele, all T31 after shooting four-under 68s.
Rahm posted a chaotic round on the South Course. The Spaniard opened with three birdies on the front nine but carded two bogeys coming in to go with three birdies and an eagle on the par-five 18th. Rahm won the US Open at Torrey Pines in 2021.
"The conditions were relatively easy. I mean, there's not much wind, but you've still got to play this golf course," said Rahm.
"I hit it great off the tee, put myself in really good positions, and the few times I was off, for the most part I was able to give myself a chance to save the hole so I think that was the key."