'I really didn't miss a shot, and made a couple good putts, but I gave myself opportunities almost every single hole, so you're bound to tap a couple in,' said the 44-year-old Cook, who hasn't made a bogey since his first hole on Thursday. 'And I finished up well, so that was a good way to end the day.'
Stuart Appleby roared out of the gate with six straight birdies en route to a 7-under 65, the low round of the day. He finished tied for second at 9-under 135 with Vijay Singh (67) and Buy.Com Tour graduate Jonathan Byrd (68).
'I missed a few very good opportunities the last two days, but I made a couple of nice ones, too,' said Appleby, an Australian who captured the last of his three PGA Tour titles in 1999. 'I probably missed more than I made.'
Appleby's 65 was his best score in 24 career rounds at Cog Hill. His previous best was a 66 in the second round of the 1999 Western Open.
Cook, an 11-time winner, is in the midst of an interesting season, his 23rd on the PGA Tour. He is currently 24th on the money list with a strange mix of six missed cuts and four top-10 finishes in 15 events. Twice he's finished second, most recently at the Memorial. His runner-up showing in Jack's tournament helped earn him a spot in the U.S. Open, where he wound up missing the cut.
This tournament wouldn't seem like the best place for Cook to bounce back. His best showing in his 13 previous Western Open appearances was a tie for 21st in 1997, and he's missed the cut twice in his last three starts at Cog Hill.
Cook faired even worse at the event's prior home, Butler National. During the seven times he played the Oak Brook, Illinois layout between 1980 and '88, Cook never registered a round in the 60s.
'I came here with no expectations,' Cook said. 'Maybe that's the key.'
Cook missed a five-foot par putt on his first hole Thursday, but has made three birdies on each nine since. And while he's hit under 70 percent of his fairways and greens thus far, Cook is No. 1 in the field in putting with only 50 putts taken through two rounds.
On Friday, Cook tied Appleby, Singh and Byrd at 9-under with a pitch to eight feet for birdie at the par-5 15th. Cook birdied the 16th to claim the outright lead, then padded his cushion by knocking a 6-iron four feet behind the hole to set up a closing three at the par-4 home hole.
Singh's name near the top of the leaderboard is a familiar site at Cog Hill. The two-time major winner from Fiji has four top-10s in the Western Open since 1996, including a runner-up finish behind Joe Durant in '98.
'For some reason I play well when I come over here,' he said. 'I like the golf course, I like the way it's set up. You have to maneuver the ball a lot over here, and for some reason I do well.'
Byrd followed up a bogey at 14 with an eagle at the 15th, where he set up his chip-in with a 6-iron that landed pin high in the thick rough.
'I was only 15 steps from the hole, and I had a kind of into-the-grain, grassy lie,' said the 24-year-old Clemson grad. 'Made good contact and it just came out perfect and trickled into the edge.'
Robert Allenby, who won the 2000 Western Open in a playoff with Nick Price, shot a 67 to tie for fifth with Durant (68), Neal Lancaster (68), Scott Verplank (69), Jerry Kelly (69) and Bob Estes (70) at eight-under.
Rocco Mediate (68), Paul Gow (68) and 2001 runner-up Davis Love III (70) finished four shots off the pace. First-round leader David Gossett (73) was part of a five-way tie at minus 6.
Last year's winner Scott Hoch turned in rounds of 72-72 to miss the cut by one stroke. He became the third defending champion in the last four years to miss the cut.
Full-field scores from the Advil Western Open