Tewell used a 6-iron to sink his second shot from 173 yards on the par-4 opening hole. It was the highlight of a round of 6-under 66 that left him at 9-under 135 for the tournament.
One stroke behind was his playing partner, Stewart Ginn, who shot 67. Tom Jenkins shot the low round of the day, a 65 that left him tied with Hale Irwin, two strokes back at 137.
The shot of the day, of course, belonged to Tewell, who started the second round tied for eighth place. Moments earlier, he had been adjusting his setup on the driving range. Suddenly, he realized how well the move worked, making eagle to climb six spots on the leaderboard.
'I equate it to NASCAR,' Tewell said. 'The car ran well after I made the adjustment.'
This is the 25th year of the Legends of Golf, the tournament largely credited with spawning the senior tour.
For the second year, the course is being played on the King & Bear Course at World Golf Village, a layout designed by Jack Nicklaus and Arnold Palmer, neither of whom are among the 'legends' in this 57-player field.
Tewell, who won four events in 25 years on the PGA Tour, doesn't consider himself among the greats.
'I can't think of much Doug Tewell has done for the senior tour other than pluck about $4 million out of it,' he said.
The winner of the Legends will receive $306,000, the largest first-place prize on tour so far this season.
Irwin is going for his third victory of the young season. He came into Saturday one stroke off the lead, and finished at 7-under after a round of 70 that could have been better.
The tour's money leader underclubbed on the par-3 14th and hit into the water. His third shot failed to make it out of the rough and he made double bogey. He finished with two birdies to work his way back up the leaderboard.
Tewell's only bogey came on the par-5 18th, when he left his lay-up second shot at an awkward distance - 48 yards - and had to ease up on a wedge to get the ball over a high bunker, yet still stop on the green. He wound up in the bunker and needed two putts to get down.
'I probably should have just gone for it on the second shot,' Tewell said. 'Everything was straight As up to there.'
Ginn, meanwhile, made birdie on No. 18, and a three-stroke advantage was trimmed to one.
'It's a game now,' Ginn said. 'Now, we've just got to go out and play golf tomorrow.'
First-round leader Ed Dougherty, hampered by a cold, shot 73 and fell four strokes behind.
News, Notes and Numbers
*Bruce Lietzke teamed with Bill Rogers to run away with the two-man, best-ball format that most of the older players competed in Friday and Saturday. Players can only enter one event, and Lietzke, 50, chose to pair with his college roommate to vie for the $48,000 first prize instead of the six-figure winner's check in the main, medal-play event.
'It's not the first time I've passed up money,' Lietzke said. 'Golf isn't in my top 10 priorities, but friendships are.'
Full-field scores from the Liberty Mutual Legends of Golf