Both the Pebble Beach and Spyglass Hill courses are being used for the tournament, which has a purse of $US2.4 million ($A3.6 million).
Six players, including Australian David Bransdon, Fijian Vijay Singh and Kiwi Steven Alker, third in the Charles Schwab Cup standings, posted five-under 67, while eight more players made 68.
Weir started on the back nine at Pebble Beach and birdied four of the first eight holes. He made the turn in 32 and got to seven under with a birdie at the par-3 seventh. But he gave that shot back one hole later and settled for a 66.
"Yeah, it was just a really solid day," said Weir, who will play Spyglass Hill on Saturday. "I played really well, I hit a lot of really close shots. My wedge game was really good, I hit a lot of close wedges to the hole. Just played really solid, so it was nice, a nice start to the week."
Percy, who is 11th on the Schwab Cup leaderboard, also started on the 10th hole at Pebble Beach.
He was a pedestrian one under after eight holes, but reached the par-5 18th hole in two and sank an eagle putt to jump-start his round. He followed that with back-to-back birdies at Nos.1 and 2 as he continues his solid 2025 season.
Asked about the course conditions, Percy said: "It's really good. I never played it this firm, so everything's bouncy. I keep waiting for it to spin back and it doesn't. I need to adjust when I come back here on Sunday."
In addition to Brandson, Singh and Alker - the latter two played Spyglass Hill on Friday - Australian Rod Pampling, Ken Tanigawa and Thailand's Thongchai Jaidee carded 67s. Jaidee won the title in 2023.
Singh had a great day on the greens, but only hit nine of 18 in regulation. He led the field with 1.44 putts per GIR.
"I changed putters and for some reason it's working really well," said Singh. "Made a couple of good putts. I hit the ball pretty good as well. My short irons were not so great, need some work."
Argentina's Angel Cabrera, who won two senior major titles earlier this year, is one of eight golfers at four under.
