Jenkins also carded a 70, as did Tom Watson, who held onto third place at 12-under-par 204. Hale Irwin (66), Tom Kite (69) and Graham Marsh (69) moved into the hunt, however, as they stand at 10-under-par 206.
Thorpe, whose lead dropped to two strokes during the third round, opened with a birdie on the par-four third at Sonoma Golf Club. He ran off five straight pars before a birdie at the ninth moved him to 16-under.
He dropped a shot at the 11th as Jenkins birdied the same hole to get to minus-13. Thorpe responded with back-to-back birdies from the 12th to get back to 17-under. He parred four holes in a row before bogeying the last when he lost his drive left for the second straight day.
'Today was a round I needed to get out of the way,' said Thorpe. 'I had a few more butterflies today and I feel like I didn't attack the course. The bogey on 18 wasn't the worst bogey in the world. It could have been a big number there.'
Thorpe could win the money title with a victory this weekend. If he wins, he needs Watson to finish worse than tied for second.
'Winning the money title would be great,' Thorpe said. 'It's something I've never done before and I've never worked as hard on my game as I have the past four or five years. I realize this is my last go-around.'
Jenkins, who aced the 17th on Friday, also opened with a birdie on No. 3. However, he stumbled to a bogey at the sixth. Jenkins erased that mistake with a birdie on the eighth.
On the back side, Jenkins climbed within two shots of Thorpe with birdies at the 11th and 13th. He stumbled to a bogey at No. 16 to finish three behind Thorpe.
'When you get closer and closer to the end, funny things happen,' said Jenkins, who won the Bruno's Memorial Classic earlier this year. 'If you're within three or four strokes going into the last nine holes, you have a good shot at it.'
Watson, who won the JELD-WEN Tradition and Senior British Open this year, birdied the third from 10 feet out. Like Jenkins, Watson bogeyed the sixth after his second sailed over the green and he was unable to get up-and-down for par.
Watson responded with consecutive birdies from the eighth to get to 12-under. He bogeyed the 15th, but came right back with a birdie at the very next hole.
'I didn't play nearly as the well today as I did the first two days,' said Watson. 'I didn't drive the ball particularly well, but I scrambled pretty well and the short game was good, because it kept me within shouting distance of Jim and I'm still in the game. If you drive the ball well you're going shoot low, if you don't, you're going to struggle, like I did today.'
Watson, who is battling for several top awards, played in all nine majors this season, five on the Champions Tour and four on the PGA Tour. He made the cut in seven of those nine events. He won two times and finished second twice, both on the Champions Tour.
'It does have a different feel (than the majors) because this is a limited field,' Watson said. 'When you play in a limited field sometimes you get a break because maybe the chances of you having to shoot your lowest score in a limited field are not as good as when you have a full field. When you have a full field, you've got to be on because there are going to be some people out there who are going to make the move every day.'
Dave Barr, Vicente Fernandez, Bruce Fleisher, Gil Morgan, Larry Nelson and Craig Stadler share seventh place at 8-under-par 208.
Isao Aoki fired a 3-under 69 to win the 54-hole Georgia-Pacific Grand Champions, which was contested by 16 players over the age of 60 and played at the same venue. Aoki finished the event at 8-under-par 208, six strokes clear of Raymond Floyd and Tom Wargo.