HOUSTON, 4 Apr 2015 (AFP) - Spieth finished with six birdies and one bogey to build a 54-hole total of 14-under 202 and was one stroke in front of Americans Scott Piercy, Johnson Wagner and Austin Cook.
Piercy, who matched the course record with a 63 on Thursday before ballooning to a 74 on Friday, rebounded with a six-under par 66. Wagner also signed for a 66 while Cook carded a two-under 70 to round out the group on 203.
Englishmen Kelvin Day and Paul Casey both shot 68s to head a group on 205, where they were joined by Charles Howell (69) and Russell Henley (68).
Phil Mickelson, whose game appeared to be coming around just in time for a run at a fourth Masters title, endured a tough day that included a double bogey and four back-nine bogeys -- three in a row to end his three-over 75.
He was in a group on 208, six shots off the pace.
But Spieth insisted that the availability of birdies on the Golf Club of Houston course meant his one-shot lead would mean little on Sunday.
"I'm not going to think about being up one, I'm going to think about playing the golf course," said the 21-year-old.
"It's a bunched leaderboard with a lot of guys making birdies. I'm not necessarily the favorite tomorrow just being up one."
If Spieth can hang on for a third US PGA Tour title, it will give him massive momentum heading into the first major of the year at Augusta National next week.
Last year Spieth led by two strokes with 11 holes to play on Sunday at Augusta, but mid-round bogeys opened the door to eventual winner Bubba Watson.
- Driver key for Spieth -
Since then he has notched victories at the Australian Open and the unofficial Hero World Challenge hosted by Tiger Woods before adding a second US Tour title at the Valspar Championship and finishing second in the Texas Open this month.
Spieth said accuracy off the tee was the key on Saturday. He hit 13 of 14 fairways in regulation and 16 of 18 greens.
"I figured starting out today that the course would play tougher with this northeasterly wind," he said. "So I put a premium on driving the ball. I drove the ball extremely well to give myself some chances coming in."
After two birdies on the front nine, Spieth stepped it up coming in.
He rolled in birdie putts of three feet and seven feet at 10 and 11, and picked up another stroke at the par-five 13th with a five-foot birdie.
A three-putt bogey at the par-three 14th was the only blemish on Spieth's card, and he pulled the shot back with a 41-foot putt from just off the green at the par-three 16th.
Leading third-round scores on Saturday in the US PGA Tour $6 million Houston Open (USA unless noted, par-72)
202 - Jordan Spieth 69-66-67
203 - Scott Piercy 63-74-66, Johnson Wagner 69-68-66, Austin Cook 68-65-70
204 - Shawn Stefani 66-69-69
205 - Kelvin Day (ENG) 68-69-68, Paul Casey (ENG) 68-69-68, Charles Howell 66-70-69, Russell Henley 69-68-68
206 - Patrick Reed 68-71-67, Keegan Bradley 70-66-70
207 - Cameron Tringale 68-70-69, Sergio Garcia (ESP) 67-71-69, Alex Cejka (GER) 65-72-70, Michael Putnam 68-68-71, Hunter Mahan 67-68-72, Luke Guthrie 66-68-73
208 - Chad Collins 69-69-70, Kevin Streelman 70-69-69, Brendon de Jonge (ZIM) 73-67-68, K.J. Choi (KOR) 71-69-68, Daniel Berger 70-70-68, J.B. Holmes 65-70-73, Andrew Putnam 67-65-76, Phil Mickelson 66-67-75