The 27-year-old started at Torrey Pines by birdying the Number 10 on the North Course of the San Diego venue, before collecting seven more birdies on a productive day.
The Spaniard, who is trying for his third win in as many starts, made a double bogey on the par-four seventh hole and was continually left frustrated on the more challenging course.
None of the three leaders has won a Tour title, while Ryder is also coming off three straight missed cuts and rookie Grant’s last four rounds have been 74 or worse.
Rai was pleased after play, admitting it had been a “very good” round and start to the tournament.
“Really drove it great off the tee, which is probably the most important thing to at least give ourselves an opportunity with those approach shots,” he said, in comments carried by the San Diego Union-Tribune.
“As soon as you miss the fairway here with anything more than a short iron (it) is very, very difficult.
“So that was key today, drove it very well. Hit a lot of real good iron shots and made a few putts around the turn there, so all in all a very good round, very good start.”
Ryder indicated he had more to accomplish, telling PGA Tour: “You’re only as good as how you finish and there’s three more days of golf.”
Rahm will be looking to bounce back, after winning four of his last six events, including his last two starts on the PGA Tour at last week’s American Express and the season-opening Sentry Tournament of Champions.
The tournament is being held from Wednesday to Saturday to avoid a clash with the NFL’s AFC and NFC championship games on Sunday.