Rahm carded four consecutive birdies and two eagles as he set the pace with a 10-under-par 62, a shot ahead of his Ryder Cup team-mate Rose.
It was also a good day’s work on the North Course at Torrey Pines for Knox, who recovered from being two-over early on to sign for a 67.
The Inverness man sparked his turnaround with an eagle at the par-5 before adding six birdies, including five on the spin around the turn.
“With all the rain they’ve had here the rough is brutal and as tough as I’ve seen it here and a few poor drives early on was the reason for the early bogeys,” Knox told golfbytourmiss.com. “From thereon I was on the fairways and able to attack.”
The effort left the Scottish No 1 in a tie for 15th and he added: “It was a little disappointing to leak the drive left down the last and walk off with a bogey but overall I have to be pleased as there was plenty of positives in my round.”
In his first regular PGA Tour start since winning the Tour Championship in September, Tiger Woods signed for a two-under 70 on the South Course, where Rory McIlroy had a 71.
“It will be a lot easier now I’ve got a competitive round under my belt,” Woods also told golfbytourmiss.com.
“As I get more comfortable, I’ll fire at more flags and start being a little bit more aggressive and start feeling a little bit more comfortable with the game and being back competing again.”