Winless this season with four runner-up finishes, Couples had a bogey-free round at TPC Harding Park.
"This is a big event," Couples said. "These are the best players on the tour and I want to win. I'm certainly not playing any better because I want to win. I want to win at Birmingham, Seattle, Chicago, Newport and everywhere else I finished second."
First-round leader Peter Senior bogeyed the 18th for a 69 to fall two strokes back.
"I didn't do much wrong," Senior said. "I didn't get away with anything but I think I'm in a good position. I had a heap of chances early in the round that I didn't convert."
Bart Bryant was third at 8 under after a 66. Bernhard Langer was another stroke back after a 68.
Charles Schwab Cup points leader Kenny Perry had a 71 to remain 3 under. Langer is the only player with a chance to overtake Perry. Even Langer wins the tournament, Perry would take the season title and $1 million annuity if he finishes fifth or better.
Fred Funk aced the par-3 eighth hole with a 4-iron, but bogeyed four of his next seven holes and finished with his second straight 70.
Couples took the lead when he parred the eighth hole and Senior had a bogey. Couples birdied Nos. 9 and 10 and led by three strokes.
"I had another good round and hit the ball well," Couples said. "When I hit pretty good irons I made the birdies. When you're near the lead, playing fast is a bonus."
Couples, who last won at the Senior British Open in July 2012, continues to deal with neck and back pain and said he'll consult a doctor Monday to find the cause.
"There's something going on," Couples said. "It's just not comfortable. It's doable. Sometimes when I turn I snag it. It's more at night when I'm lying down. My back hurts and I don't sleep much. I wouldn't be playing if it hurt to swing."
Couples learned how to play golf at a public course in Seattle, which is why he feels comfortable at Harding Park.
"This is one of my all-time favorite courses," Couples said. "That helps me. As far as length, if I hit every fairway, I'll have an advantage all right, over my normal game."
Senior, who spent most of his career on the PGA Tour of Australasia, has yet to win on American soil.
"If you play well enough, you're going to do it," Senior said. "Inside me I know I'm good enough to win tournaments and I'll go out there with that on the weekend."