Horschel shot a bogey-free nine-under 63 on the North Course and led Michael Thompson by one stroke after the opening round of the US PGA Tour event.
Of the 30 players who shot 67 or better, 26 played the North. Rahm's was the best score on the South by one over Luke List, Peter Malnati and Cameron Tringale. The players switch courses on Thursday and play the final two rounds on the South.
Cam Davis led the Australians with his four-under 68 on the North Course, while Jason (South) shot 70 and Marc Leishman (South), Matt Jones (North) and Brett Drewitt (North) all carded 71s.
Taking advantage of the North's shorter distances and more forgiving rough, Horschel made five birdies on the front nine.Â
"I told my caddie, 'Let's just go out there, no expectations, and just enjoy,'" Horschel said. "And I think we did a really good job of that. My game is in a really good spot."
Thompson also played bogey-free, putting him one shot ahead of Stephan Jaeger and Kevin Tway.
Rahm's strong start in San Diego is no surprise: The world No.1 got his first PGA Tour victory at Torrey Pines in 2017, and he won last year's U.S. Open on the South Course for his first major.
Rahm said the conditions were "relatively easy," even on the South. He stumbled with two bogeys on the back nine before finishing strong, crushing a 280-yard approach shot on the par-5 18th to 12 feet for eagle.
"It was probably one of the best swings I'll make all year," Rahm said. "That 3-wood was absolutely perfect."
The Farmers Insurance Open being played Wednesday through Saturday to avoid a television conflict with the NFL's two conference championship games on Sunday.
