McIlroy rebounded from an early bogey with two birdies on his outward nine, then picked up three more shots after the turn to climb into the upper reaches of the leaderboard in the idyllic surrounds of Silverado Country Club.
"I felt like I played pretty well," four-times major winner McIlroy told Golf Channel. "I got off to a bit of a slow start this morning but gradually got into the round and started hitting some better shots and giving myself plenty of chances.
"I didn't convert many of them but still a pretty solid score and in good position going into the next three days."
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McIlroy missed three birdie putts from inside 10 feet after reaching 17 of 18 greens in regulation but was delighted to achieve his pre-tournament goal of getting into contention for a fourth victory this year.
His three previous wins came before the end of May and he was sidelined for almost two months after rupturing a ligament in his ankle while playing football with friends on July 4.
"I want to have chances to win golf tournaments, that's really my focus until the end of the year," said McIlroy. "And it's important to get a good start (to the 2015-16 PGA Tour).
"The last couple of years ... I feel like I have always been playing catch-up because I usually start my PGA Tour season at the end of February and most guys are starting now.
"So it's nice to get a couple of events in before the turn of the year. Hopefully when I get back to Florida (in late February) I won't be feeling the pressure of trying to catch up to these guys."
With a grin on his face, McIlroy suggested that his biggest challenge this week would be trying to remain in contention while also making the most of his first trip to the Napa Valley.
"I'd never been to this area before but you can see why everyone is relaxed. Beautiful weather and there's a lot of stuff to do away from the golf course," the 26-year-old said.
"So I'm trying to find a balance between enjoying myself this week and still trying to play well. It's a great event and I'm obviously very happy to be here."
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)
