Singh only managed a 1-under 70 on Friday, while Jacobson was a stroke better with a 69. Donald, the Englishman who won the 1999 NCAA Individual Championship at Northwestern, carded a 5-under 66.
Tom Byrum earned low round of the day honors with a 7-under 64. He is alone in fifth place at 7-under-par 135.
Couples started on the back nine at Westchester and promptly landed a sand- wedge 8 feet from the hole to set up birdie. He added another birdie at the 154-yard, par-3 14th when his 9-iron stopped 3 feet from the hole.
Couples, the 1992 Masters champion, sank a 10-footer to save par at the 16th then wedged his approach to two feet for a tap-in birdie at the 17th. He made a pair of nice up-and-downs for par at 18 and one, then waited to take advantage of the par-5 fifth.
Last year's Houston Open champion reached the front fringe with his second and two-putted for birdie from 50 feet. Couples played an 8-iron to 8 feet for a birdie at the eighth that tied for the lead after Jacobson, in the group ahead of Couples, made a mess on No. 9.
Couples hit a long drive at the par-5 ninth, then nailed a 3-iron over the green. He hit a spectacular chip to 2 feet and kicked in the birdie putt to take his first 36-hole lead since last year in Houston.
'Today I really hit the ball well, had a lot of birdie putts,' said Couples. 'I've kept the ball in play, which is a key here. I hit a lot of good shots, made a few key par putts yesterday and today to keep up there and keep going.'
Couples long history of back problems has resurfaced this week, but he's been getting through it, thanks in part to a new treatment. He has received acupuncture treatments before Thursday's opening round and before Friday's second round.
'This back thing, it's old for me,' said Couples. 'I just learned how to get my body to swing, and I am swinging pretty well and things are going well, so it's really not a big deal. My back is rotating and moving, so it's a good thing. It's just hard when you have this throbbing and you're not real used to it.'
Singh, who narrowly missed the course record Thursday with an 8-under 63, wasted little time in breaking into red numbers. He knocked a pitching-wedge to 10 feet to set up birdie at the second. Singh parred his next nine holes before a miscue at the 12th.
The No. 3 player in the world hit a 5-iron into a greenside bunker at the 473-yard hole. Singh blasted out to 8 feet and missed the par save. He parred his next five then two-putted from 20 feet at the last to polish off his round of 70.
'I didn't hit it as close as yesterday, but hit as many greens,' said Singh, who has hit 17 of 18 greens in regulation in each round. 'But I'm really happy the way I'm playing.'
Jacobson was stagnant at even par through his front nine, the back side at Westchester. Things turned at the second when the Swede holed a wedge from 112 yards out for an eagle.
Jacobson was not done yet. He ran home a 10-footer for birdie at the fifth, then added a 15-foot birdie putt at the sixth to go to 11 under par, two clear of Singh.
But Jacobson struggled from the beginning at the par-5 ninth when his drive went left into the trees. The Swede got a great break when his ball kicked out, but he had to lay up short of the green. Jacobson hit a 9-iron right of the green and the ball came to rest on the collar. His explosion shot did not work as the ball went to the other fringe and Jacobson chipped 5 feet past the hole. Jacobson missed the bogey putt and took a double bogey, falling out of first into the tie for second.
'I'm going to be a little bit disappointed now, but as I tee up tomorrow, I'll still realize that I'm in a great position,' said Jacobson, ranked 23rd in the world. 'I'm in position where if I play well tomorrow I can win the tournament on Sunday.'
Donald, who played in the same group as Couples, tallied only one birdie over his first 10 holes, but birdied three of his next four holes from the second. His fairway-metal at the ninth stopped 25 feet short of the hole, but he missed the eagle putt at tapped in for birdie.
'I played nicely the last couple of days,' said Donald. 'I think I've been struggling with my putting the last couple of weeks, but the last couple of days I seem to have found something and I'm putting very nicely and that's kind of the key to how I've scored the last couple of days.'
Padraig Harrington (68), Scott Hend (68), Billy Mayfair (68), Tjaart van der Walt (65) and Cameron Beckman (68) are tied for sixth place at 6-under-par 136.
Phil Mickelson, Ernie Els, Sergio Garcia and Miguel Angel Jimenez are part of a logjam in 11th at minus-5.
Defending champion Jonathan Kaye rebounded from an opening-round 73 with a 5-under 66. He is part of a large group of golfers tied for 35th at minus-3.
The 36-hole cut fell at 1-under-par 141 and among the notable players who failed to make the weekend were: Mike Weir (142), Chad Campbell (142), Jay Haas (143) and Darren Clarke (143).