"I tapped in the 7-iron from 188 (yards)," said Cook, who was at 11-under 133. "It hasn't happened to me in a while. It was a nice surprise."
He made the eagle on the 496-yard 12th, which played as the 31st toughest hole on tour last year with an average score of 4.33.
Cook had six birdies to go with the eagle in his bogey-free round at the 7,552-yard Montreux Golf & Country Club and had several rising stars on his heels.
Former NCAA champion Charles Howell III shot a 66 and was at 134 along with David Berganio and Jerry Kelly, who both shot 68s.
Bryce Molder, a four-time All-American at Georgia Tech making his pro debut, shot a 65 and was in a group of eight at 135 along with two-time U.S. Open champion Lee Janzen and first-round leader Brian Watts, who had a 71.
The 43-year-old Cook is 28th on the career money list with $8.7 million but he has topped the $1 million mark only once in the last 10 years. In 1992, he was second in the British Open and tied for second at the PGA Championship. The next year he tied for sixth at the PGA and in 1994 he tied for fifth at the U.S. Open and tied for fourth at the PGA.
He's won $420,000 on tour this year, 89th on the money list, with his best showing a tie for fourth at the Shell Houston Open in April.
"It's been a struggle the last few years but the last 12 months I started to hit the ball well and it's starting to show," Cook said.
"There's a lot of kids there who are pretty thirsty, pretty hungry. We crusty veterans sometimes can sneak up there. It's one for the old guys this week."
The 22-year-old Howell, who turned professional last year after winning the NCAA title at Oklahoma State, has three top 10 finishes this year, including a second in a playoff at the Greater Milwaukee Open.
"It's getting closer. I've been close a few times," Howell said. "If I could win a tournament, that would be awesome. It would mean everything. ... It would do a lot of things for my status on tour."
Berganio, 32, also has three top 10 finishes this year, his best year since turning pro in 1993 out of the University of Arizona. He shot a 64 in the third round to tie for third at the Canon Greater Hartford Open in July.
"I've just been playing smarter all year," said Berganio, who bogeyed his last hole.
Molder, who turned pro after playing for the U.S. Walker Cup team in Georgia earlier this month, was 5-under for his last nine holes Friday.
"I knew if I played well I could be right there," said Molder, whose collegiate scoring average of 70.69 broke Tiger Woods' record of 71.10, which he set at Stanford.
"I came to try to win and I'm in a position with two rounds left to go to do so," he said.
Janzen got to 10-under, but missed an 8-foot birdie putt on the 636-yard, par-5 17th and took three putts to get down from 20 feet on the fringe on 18.
Mark O'Meara started the round just two strokes off the lead, but shot a 72 Friday.
LPGA Betsy King Classic
KUTZTOWN, Pa. _ Moira Dunn shot a 6-under-par 66 and held a one-stroke lead over Wendy Doolan after two rounds of the LPGA's Betsy King Classic.
Dunn, seeking her first tour win, had seven birdies Friday and was at 10-under 136 through two rounds on the course at Berkleigh Country Club. She began the day three strokes behind first-round leader Heather Daly-Donofrio.
Doolan's 67 left her at 135, one stroke ahead of Daly-Donofrio, who had a 71, and Mhairi McKay, who shot a 67.
Rachel Teske, the 1998 champion, had three birdies on her front nine and took the lead at 9 under, but she had a double-bogey on the back and finished with a 71. She was tied at 137 with Audra Burks, who had a 68.
MISSISSAUGA, Ontario _ Jim Thorpe followed his course-record opening round with a scrambling 2-under-par 69 and took a two-stroke lead in the AT&T Canada Senior Open.
Thorpe, coming off a record-tying 63, had four birdies and two bogeys to finish 36 holes on the historic Mississaugua Golf and Country Club course at 10-under 132.
Thorpe birdied the first two holes to reach 10 under, but dropped back with bogeys on Nos. 4 and 8. He regained the lead with birdies on the par-5 12th and 13th.
Walter Hall was second after a 66, and Bruce Fleisher birdied the final four holes for a 66 to join Howard Twitty (68), Mike McCullough (68) and Mike Smith (66) at 7 under. Tom Kite (70) and Danny Edwards (69) were 6 under.
ATLANTA _ Defending champ Jeff Quinney was stunned by former college teammate Brian Nosler, who won the final three holes for a 1-up victory in the quarterfinals of the U.S. Amateur Championship at East Lake Golf Club.
Seventeen-year-old Daniel Summerhays fell short in his bid to become the youngest winner in the tournament's 101-year history. The Farmington, Utah, teen-ager was eliminated 4 and 3 by Robert Hamilton of Carmichael, Calif.
Tiger Woods was 18 when he won the first of three straight amateur titles in 1994.
Nosler, 22, will meet Bubba Dickerson of Hilliard, Fla., in the semifinals Saturday. The 20-year-old Florida junior was a 1-up winner over Bermuda's Michael Sims.
In the other semifinal, Hamilton will meet Manuel Merizalde of Colombia,
who cruised to a 6 and 5 triumph over Japan's Taichiro Kiyota.