Thorpe's 63 tied the 18-hole tournament record set three previous times on three different courses. Bob Charles first set the mark in 1991 at Hyatt Dorado Beach. It was later matched by Jay Sigel in 1994 at The Dunes Golf & Beach Club and Bruce Lietzke in 2001 at Gaillardia Golf & CC.
Tom Jenkins stands alone in second place at 6-under-par 66. Defending champion Tom Watson and Larry Nelson are two strokes further back at 4-under-par 68.
Thorpe birdied the par-4 third to get his round going at Sonoma Golf Club. He later birdied the par-3 seventh and came right back with an eagle on the eighth to make the turn at minus-4.
Around the turn, Thorpe dropped in a birdie at the par-4 10th and came right back to birdie the very next hole. He birdied the par-5 13th to climb to 7 under.
Thorpe, who won the Long Island Classic earlier this year, extended his lead with consecutive birdies from 16th to cap his bogey-free round.
'It was a good day. I knew coming in this week being fifth or sixth on the money list I would get marquis pairing,' said Thorpe. 'I'm playing with probably two guys (Tom Kite and Hale Irwin) that are going to push you to play. Unfortunately, Hale didn't have a good round today, but we know he can shoot 62 or 63 tomorrow or the next day just as well as he shot 72 or 73 today.'
With many season-ending awards and the tour money title still up for grabs, Thorpe has put himself in a good position. If the top-5 remains throughout the event, Thorpe would win the money title by less than $40,000 over Watson.
'This year I came out to try and push myself as hard and as high as I could really push,' said Thorpe. 'If I win the golf tournament I have a chance to win money title which is something I've never done, I think it would be a great accomplishment with the level of competition we have out here.'
Jenkins, who captured the Bruno's Memorial Classic earlier in the year, opened with a birdie on the par-4 sixth, but stumbled to a bogey on the very next hole. He later birdied the ninth to make the turn at minus-1.
The 55-year-old rolled off four consecutive birdies from the 11th to vault up the leaderboard. He closed out his round with a birdie at the par-5 16th.
'It was a good day. After my performance last week in San Antonio, I wasn't too sure how I was going to play here,' said Jenkins. 'I started off making some routine pars early in the round. I wasn't feeling very comfortable with my game, with my swing and then I started hitting some good shots. I think the birdie putt at nine kind of turned the day around.'
Watson, who won two majors on the Champions Tour, has positioned himself well, too. Watson, who played in all nine majors on the PGA and Champions Tours combined, is battling Craig Stadler and Bruce Lietzke for Player of the Year honors.
He got off to a quick start with a birdie at the first. However, Watson dropped off the pace with bogeys at the sixth and eighth. He righted the ship with a birdie at the ninth to head to the back nine at even par.
He eagled the par-4 11th when he holed a pitching-wedge. Watson moved to minus-3 with a birdie at the par-5 13th.
Watson stumbled to a bogey at the par-3 14th. However, he eagled the par-5 16th to move into a share of third place.
'I had a very good round today,' Watson said. 'I didn't make any putts. I holed a wedge shot for an eagle at No. 11, but I had the ball close to the hole a lot today and I couldn't read the putts.
'Maybe I'm talking myself into it because yesterday I said these greens have subtle breaks to them and they are hard to read. Both caddy Bruce (Edwards) and I made mistakes out there today. We're not reading them very well. I left a lot out there on the golf course.'
Stadler, who won the SBC Championship last week, is one stroke behind Watson at 3-under-par 69. He is joined there by Kite, Dave Barr, Vicente Fernandez, Bruce Fleisher, Gil Morgan and Bobby Wadkins.