The world number six carded his second successive round of 65 to move onto 17-under-par for the tournament, as he goes in search of his ninth triumph on the PGA Tour.
The 23-year-old's round included eight birdies and just one bogey, while he only needed ten putts over the back nine as he pulled clear of the chasing pack.
Brandt Snedeker is Spieth's nearest challenger on 11-under, after he carded a round of 67 that included six birdies on the front nine, while US Open champion Dustin Johnson is a shot further back alongside Kelly Kraft.
Spieth knows he cannot afford to take his foot off the gas given the quality of the chasing pack, while he is also expecting to face a stiff test on the greens of the Pebble Beach Golf Links.
Asked what his toughest challenge will be on Sunday, Spieth said: “Recognizing these greens are very tough to putt and from inside 10 feet, there are going to be mistakes that are made. … Staying really patient and staying focused on a goal.
“Telling myself, ‘Hey, I’m not sure what it’s going to be, it might be 3 under, it might be 4 under tomorrow. But staying really focused on I’m challenging my goal, not looking at anybody else. That’s the toughest part tomorrow.”
World number one Jason Day did hold a share of the lead alongside Spieth heading into the third round, but he had a day to forget, as he carded a score of 75 to fall ten shots off the pace.