Gary McCord posted a 66 to join D.A. Weibring in a tie for third at 4-under-par 140. Mike Reid was four shots back at 3-under-par 141 while opening-round leader Isao Aoki finished alone in sixth place at 2-under-par 142.
Fergus hit a 5-iron within a foot of the cup for a tap-in birdie on the opening hole at Valencia Country Club but he found trouble with a bogey at the par-4 fourth. He responded with a birdie at the fifth and landed his approach inside 10 feet for a birdie at the par-4 eighth.
'I like this golf course,' said Fergus. 'The greens are fun to putt.'
The 51-year-old two-putted for a birdie at the par-5 ninth and ran home a 22-foot putt for a birdie at the par-4 11th. Fergus then landed his second shot within 2 feet of the hole for a birdie at the par-4 12th.
Fergus continued the birdie barrage down the stretch and got to 6 under with a two-putt birdie at the par-5 15th. He then dropped his tee shot inside 7 feet at the 16th and converted the putt for his eighth birdie of the day.
Fergus was in trouble at the par-4 17th but he rolled in a 15-foot putt to save par. He then sank a 6-foot par save at the last to match his career low on the Champions Tour.
'I hit the ball well early and made some putts,' said Fergus, who is seeking his first career victory on the over-50 circuit. 'Just try to draw on what you've done before. You have to be committed out here. These guys are good.'
McNulty birdied each of his first two holes and rattled off eight straight pars before his second shot to the 10th stopped 20 feet from the hole for another birdie.
The Zimbabwe native, who won three times on the Champions Tour last year, picked up a birdie at the 12th and played a wedge to five feet for a birdie at the 13th. McNulty then birdied the par-3 14th to make it three in a row.
'I'm in a position we'd all like to be in going into the last round,' said McNulty, who posted a 66. 'I'm delighted with my round and look forward to tomorrow.'
Bob Gilder, Lanny Wadkins, Don Pooley and Des Smyth share seventh place at 1-under-par 143.
Craig Stadler, Wayne Levi, Morris Hatalsky, Vance Heafner and Tom McKnight were one shot further back at even-par 144.